fix: close what nine adversarial reviews found, most of it mine

Nine focused reviews (export state machine, upload path, auth/abuse, client
queue, guest UI, database, deploy/ops, regression hunt, test honesty). Every
finding below was re-verified against the code before being acted on; several
plausible-sounding ones were checked and rejected.

## Data loss and denial of service

**One request could OOM-kill the app container.** `client_upload_id` was read
with `Field::text()` — axum builds its multipart reader with no SizeLimit, so
the only bound was the route's 576 MiB body limit, then decoded into a second
full String. `caption` and `hashtags` go through `read_text_field_bounded` for
exactly this reason; this field arrived later and missed it. Any guest, one
request, and every SSE stream drops and every in-flight temp file is stranded.

**Nothing bounded concurrent upload bodies.** The headroom gate can only refuse
to COMMIT — the body is already streamed to a temp file by the time it runs, and
neither axum, the tower stack nor Caddy limits how many stream at once. ~100
guests tapping "upload all" after the ceremony puts 10-20 GB of .tmp on a 40 GB
volume, invisible to the gate, eating the reserve that keeps Postgres able to
write WAL. New `UploadAdmission` budgets bytes (not requests, so one video and
two hundred photos coexist) via a permit that releases on drop, so every exit
path returns it.

**The export decode bypassed the memory permit the compression path takes.**
Same class of work — decode + resize every image in the gallery — in a bare
spawn_blocking. A release fired while the last photos were still compressing put
both in the same 1 GiB cgroup; the OOM kill marks the export failed and
`recover_exports` re-spawns it into the same conditions on the next boot. The
permit is now process-wide in `imaging`, because the constraint it expresses is
the container's memory, not one worker's.

**`MediaTotalCache` cached its own failure as 0.** For the whole TTL the gate
then saw an empty event and collapsed to the flat reserve — the behaviour the
two-halves design replaced — with no log line. And the trigger correlates with
the danger: with max_connections 10 the query fails exactly during a burst. Now
falls back to the last good reading and says so.

**V8's heap ceiling sat above the frontend container's entire budget** (measured:
259 MB inside a 256M limit), so GC could never intervene and the only
backpressure was SIGKILL under an arrival burst.

## Guest-visible

**The feed stopped being newest-first after the first reconcile.** It fetches
whole 100-item server pages while `uploads` grows in 20s, so everything in the
gap was absent from `present`, classified as new, and prepended — ~80 photos
from earlier in the evening above the newest ones. It also stalled infinite
scroll, since the cursor still pointed at item 20 and the observer only re-fires
on a change. The union is now sorted on the server's own (created_at, id) key,
which additionally places an SSE arrival correctly.

**A stale `loadMoreError` outlived every refresh and filter change**, leaving a
false error above a button that returns immediately on `!nextCursor`.

**A failed derivative toasted "Ein Upload konnte nicht verarbeitet werden."** for
a photo sitting right there on screen — the handler still assumed 1d9fb11's
pre-fix behaviour (row deleted, quota refunded, card evicted), none of which is
true any more. It was the last surviving route for the "your photo is gone"
signal that fix set out to remove.

## Enforcement that existed only in comments

`recover_name_rate_per_15min` is clamped at the point of use: the ordering
`3 x ceiling <= PIN_LOCK_THRESHOLD` is the whole control against one source
locking any guest whose name is on the feed, it was asserted in a comment, and
`patch_config` accepted 1..100_000. The test pinned the default constant rather
than the enforced bound; it now pins the bound.

## Tests that could not fail

- The gate test asserted only its own premise (`500MB x 100 > 35GB`) and never
  touched the gate. It now checks both controls against the same state and
  requires them to disagree in the right direction.
- `the_banner_always_fires_before_the_upload_gate_closes` reduced to
  `G < G + G/4` — true for any margin, including zero, so it could not detect
  the banner moving to exactly the gate. It now pins the gap.
- `disk_is_low`'s `free < LOW_DISK_FLOOR_BYTES` clause was unreachable (warn_at
  is always >= 12.5 GB against a 10 GB floor). Two tests were named after it and
  neither could fail if it were deleted. Clause and constant removed.
- The suspension test I added last commit hard-coded the credit cap instead of
  importing it, so changing STALL_TIMEOUT_MS would leave it passing against a
  system that no longer exists. Now imports MAX_SUSPEND_CREDIT_MS.

## Stale comments corrected

The prune doc still argued at length for the pre-build ordering that 1d9fb11
reversed — a reader trusting it would reopen the blocker 0506369 fixed.
DISK_RESERVE_BYTES claimed to equal the banner threshold that 0506369
deliberately offset by 25%. And host.rs kept its own duplicate 10 GB literal
instead of importing the constant.

154/154 backend, 59/59 vitest, clippy clean, svelte-check 0 errors, eslint
clean, both builds, compose + caddy validate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/services/compression.rs

@@ -13,9 +13,6 @@ use crate::state::SseEvent;
 #[derive(Clone)]
 pub struct CompressionWorker {
     semaphore: Arc<Semaphore>,
-    /// Serialises the memory-heavy image jobs — see `HEAVY_IMAGE_BYTES`. Separate from
-    /// `semaphore` so ordinary photos keep full concurrency.
-    heavy: Arc<Semaphore>,
     pool: PgPool,
     media_path: PathBuf,
     sse_tx: broadcast::Sender<SseEvent>,
@@ -34,7 +31,6 @@ impl CompressionWorker {
     ) -> Self {
         Self {
             semaphore: Arc::new(Semaphore::new(concurrency)),
-            heavy: Arc::new(Semaphore::new(1)),
             pool,
             media_path,
             sse_tx,
@@ -308,19 +304,6 @@ impl CompressionWorker {
     /// saving rather than risk the OOM kill.
     const OXIPNG_MAX_PIXELS: u64 = 8_000_000;
 
-    /// Estimated peak heap above which an image job takes the exclusive `heavy` permit.
-    ///
-    /// `compression_concurrency` (default 2) bounds how many jobs run at once, but says
-    /// nothing about how much memory each one costs, and the container gets 1 GiB total. A
-    /// single 8000x8000 original measures ~516 MiB peak even with the decode correctly scoped
-    /// — two of those overlapping is 1032 MiB and another OOM kill, from nothing more exotic
-    /// than two guests uploading big photos at the same moment.
-    ///
-    /// 150 MiB sits far above a normal phone photo (a 12 MP JPEG costs ~50 MiB all-in) so the
-    /// common path never serialises, and far below the point where two jobs stop fitting.
-    /// Throughput is unaffected for everything except the rare giant, which is exactly the
-    /// case that must not run in parallel with another giant.
-    const HEAVY_IMAGE_BYTES: u64 = 150 * 1024 * 1024;
 
     /// Wall-clock ceiling for one oxipng run.
     ///
@@ -356,13 +339,13 @@ impl CompressionWorker {
         let estimate =
             crate::services::imaging::estimated_processing_peak_bytes(&original, Self::DISPLAY_MAX_EDGE);
         let _heavy_permit = match estimate {
-            Some(bytes) if bytes > Self::HEAVY_IMAGE_BYTES => {
+            Some(bytes) if bytes > crate::services::imaging::HEAVY_IMAGE_BYTES => {
                 tracing::debug!(
                     %upload_id,
                     estimated_mib = bytes / (1024 * 1024),
                     "waiting for the heavy-image permit"
                 );
-                Some(self.heavy.acquire().await)
+                Some(crate::services::imaging::HEAVY_IMAGE_PERMITS.acquire().await)
             }
             _ => None,
         };
@@ -768,7 +751,7 @@ mod tests {
         )
         .expect("header readable");
         assert!(
-            ordinary_peak <= CompressionWorker::HEAVY_IMAGE_BYTES,
+            ordinary_peak <= crate::services::imaging::HEAVY_IMAGE_BYTES,
             "a 12 MP photo estimated at {} MiB would serialise the common path",
             ordinary_peak / 1048576
         );
@@ -782,7 +765,7 @@ mod tests {
         )
         .expect("header readable");
         assert!(
-            giant_peak > CompressionWorker::HEAVY_IMAGE_BYTES,
+            giant_peak > crate::services::imaging::HEAVY_IMAGE_BYTES,
             "an 8000x8000 RGBA original estimated at only {} MiB would be allowed to run \
              concurrently with another one — 2x its real ~516 MiB peak does not fit in 1 GiB",
             giant_peak / 1048576

backend/src/services/export.rs

@@ -522,6 +522,21 @@ async fn ensure_export_space_reclaiming(
     ensure_export_space(pool, event_id, export_path).await
 }
 
+/// Take the process-wide heavy-image permit if this file is big enough to need it.
+///
+/// Mirrors the compression worker's gate exactly (a header probe, no pixels decoded), so the two
+/// producers of heavy image work agree on what "heavy" means and serialise against each other
+/// rather than each against itself.
+async fn heavy_permit_for(path: &Path) -> Option<tokio::sync::SemaphorePermit<'static>> {
+    let estimate = crate::services::imaging::estimated_processing_peak_bytes(path, 2048);
+    match estimate {
+        Some(bytes) if bytes > crate::services::imaging::HEAVY_IMAGE_BYTES => {
+            crate::services::imaging::HEAVY_IMAGE_PERMITS.acquire().await.ok()
+        }
+        _ => None,
+    }
+}
+
 // ── ZIP export ───────────────────────────────────────────────────────────────
 
 async fn run_zip_export(
@@ -888,6 +903,12 @@ async fn run_html_export_inner(
             let thumb_path = media_tmp.join(&thumb);
             let thumb_path_clone = thumb_path.clone();
 
+            // Same process-wide memory permit the compression worker takes. Without it, a
+            // release fired while the last phone photos were still compressing put an export
+            // decode and a heavy compression job in the same 1 GiB cgroup — and the OOM kill
+            // marks the export failed, which `recover_exports` then re-spawns into the same
+            // conditions on the next boot.
+            let _heavy = heavy_permit_for(&src).await;
             let thumb_result = tokio::task::spawn_blocking(move || -> Result<()> {
                 // `decode_oriented`, not `image::open`: the latter ignores the EXIF
                 // orientation tag AND applies no decode limits. Using it here is why every
@@ -922,6 +943,9 @@ async fn run_html_export_inner(
                 let full_path = media_tmp.join(&full);
                 let full_path_clone = full_path.clone();
 
+                // See the thumbnail above. This branch is the more expensive of the two: it
+                // only runs for originals over 5 MB, i.e. exactly the giants.
+                let _heavy = heavy_permit_for(&src).await;
                 let compress_result = tokio::task::spawn_blocking(move || -> Result<()> {
                     // Same reason as the thumbnail above. This branch only runs for originals
                     // over 5 MB, which is why the viewer's full image looked correct for small
@@ -1315,16 +1339,19 @@ async fn protected_files(pool: &PgPool, event_id: Uuid) -> Vec<String> {
     .unwrap_or_default()
 }
 
-/// Reclaim superseded FINAL archives BEFORE this generation starts writing its own.
+/// Reclaim superseded FINAL archives.
 ///
-/// Peak disk usage used to be two full generations, because the only prune ran after the new archive
-/// was written, renamed and finalised. That ordering reads as durability ("don't delete the good
-/// keepsake before the replacement is safe") but it buys nothing: readiness is derived from
-/// `job.epoch = event.export_epoch AND status = 'done'`, so the moment `invalidate_and_arm` bumps
-/// the epoch the old archive is ALREADY unreachable — `GET /export/zip` 404s whether the file is on
-/// disk or not. Keeping it only reserves gigabytes for a download nobody can perform, and for
-/// `Affects::Both` (a takedown) it is content someone has explicitly asked to have removed. So a
-/// rebuild reclaims first and peaks at one generation.
+/// CALLED AFTER A SUCCESSFUL BUILD, not before one. This doc used to argue the opposite at
+/// length — that since readiness is derived from `job.epoch = event.export_epoch`, a superseded
+/// archive is already unreachable and keeping it "buys nothing". That reasoning is right about
+/// REACHABILITY and wrong about RECOVERABILITY: an epoch is a database value that can be rolled
+/// back, deleted bytes cannot. Pruning first meant any rebuild that then failed — ENOSPC, an OOM,
+/// a hung ffmpeg — left the event with NO archive at all, which is the one outcome the product
+/// exists to prevent, at the one moment nobody is watching.
+///
+/// The single exception is phase 2 of `ensure_export_space_reclaiming`, where the previous
+/// generation's bytes are the only way the rebuild can fit at all. Both call sites carry the full
+/// reasoning; do not "restore" a pre-build prune on the strength of this function's convenience.
 ///
 /// Narrower than [`prune_stale_export_files`] on purpose: FINAL archives only. Those are inert — a
 /// superseded worker either already renamed its file (and will delete it itself when its guarded

backend/src/services/imaging.rs

@@ -205,6 +205,32 @@ pub fn decode_oriented(path: &Path) -> Result<DynamicImage> {
     Ok(img)
 }
 
+/// Process-wide serialisation for memory-heavy image work.
+///
+/// The `app` container gets 1 GiB. A single 8000x8000 original measures ~516 MiB peak even with
+/// the decode correctly scoped, so two overlapping giants is an OOM kill — and the kernel kills
+/// the whole process, dropping every SSE stream and stranding every in-flight upload.
+///
+/// GLOBAL rather than a field on `CompressionWorker`, because the constraint is the container's
+/// memory and there is more than one producer of this work. The export's own image path
+/// (`services::export`) decodes and resizes every photo in the gallery — a thumbnail for each,
+/// plus a 2000px re-encode for every original over 5 MB — and it ran in a bare `spawn_blocking`
+/// with no permit at all. So "host taps Freigeben while the last phone photos are still
+/// compressing" put an export decode and a heavy compression job in the same cgroup at the same
+/// time, which is the scenario the permit exists to make impossible. Worse, it is self-repeating:
+/// the OOM kill marks the export failed, and `recover_exports` re-spawns it on boot into the same
+/// conditions.
+///
+/// Held across the blocking section and released on drop, including on error.
+pub static HEAVY_IMAGE_PERMITS: std::sync::LazyLock<tokio::sync::Semaphore> =
+    std::sync::LazyLock::new(|| tokio::sync::Semaphore::new(1));
+
+/// Estimated peak heap above which a job must take [`HEAVY_IMAGE_PERMITS`].
+///
+/// 150 MiB sits far above a normal phone photo (a 12 MP JPEG costs ~50 MiB all-in) so the common
+/// path never serialises, and far below the point where two jobs stop fitting in the container.
+pub const HEAVY_IMAGE_BYTES: u64 = 150 * 1024 * 1024;
+
 #[cfg(test)]
 mod tests {
     use super::*;

backend/src/services/media_total.rs

@@ -61,15 +61,37 @@ impl MediaTotalCache {
         {
             return bytes;
         }
-        let bytes = sqlx::query_scalar::<_, Option<i64>>(
+        let queried = sqlx::query_scalar::<_, Option<i64>>(
             "SELECT SUM(total_upload_bytes)::bigint FROM \"user\"",
         )
         .fetch_one(pool)
-        .await
-        .ok()
-        .flatten()
-        .unwrap_or(0)
-        .max(0);
+        .await;
+
+        let bytes = match queried {
+            Ok(v) => v.unwrap_or(0).max(0),
+            Err(e) => {
+                // FAIL OPEN, but do NOT cache the failure, and do NOT let it pass silently.
+                //
+                // Storing 0 here pinned the gate's view of the event at "empty" for the whole
+                // TTL. During that window `media_after` is just this upload, `keepsake_needs`
+                // collapses to ~2.2x one file, and the gate degrades to the flat 10 GB reserve —
+                // precisely the behaviour the two-halves design replaced, reappearing with no
+                // trace in the log. And the trigger correlates with the danger: with
+                // `max_connections = 10` and a 5s acquire timeout, this query fails exactly when
+                // a burst is in progress.
+                //
+                // Falling back to the LAST GOOD reading (however stale) is strictly better than
+                // 0: the total only ever grows, so a stale value under-counts slightly, while 0
+                // under-counts by everything.
+                let previous = self.inner.read().unwrap().map(|(b, _)| b);
+                tracing::warn!(
+                    error = %e,
+                    fallback_bytes = previous.unwrap_or(0),
+                    "media total query failed; upload gate is running on a stale reading"
+                );
+                return previous.unwrap_or(0);
+            }
+        };
         *self.inner.write().unwrap() = Some((bytes, Instant::now()));
         bytes
     }

backend/src/services/mod.rs

@@ -7,4 +7,5 @@ pub mod maintenance;
 pub mod media_total;
 pub mod rate_limiter;
 pub mod sse_tickets;
+pub mod upload_admission;
 pub mod video;

backend/src/services/upload_admission.rs

@@ -0,0 +1,155 @@
+//! Admission control for upload bodies, budgeted in BYTES rather than requests.
+//!
+//! ## Why this has to exist
+//!
+//! The keepsake headroom gate in `handlers::upload` cannot bound a burst, and the reason is
+//! structural rather than a bug in the gate: the request body is streamed to a temp file during
+//! multipart parsing, so the bytes are already on disk by the time any check runs. The gate can
+//! only refuse to COMMIT them. Nothing upstream limited how many bodies stream at once — axum has
+//! no such limit, the tower stack is just `TraceLayer`, and Caddy passes requests straight
+//! through.
+//!
+//! So the failure mode is the ordinary one, not an attack: the ceremony ends, ~100 guests tap
+//! "upload all", and ~100 bodies stream concurrently. At phone-video sizes that is 10-20 GB of
+//! `.tmp` files on a 40 GB volume, none of it visible to the gate, and `DISK_RESERVE_BYTES` — the
+//! 10 GB standing between the party and Postgres losing the volume it writes WAL to — is consumed
+//! by transient files. The `.tmp` sweeper only reclaims files idle for an hour, correctly, which
+//! means nothing reclaims a burst on this timescale.
+//!
+//! ## Why bytes and not a request count
+//!
+//! A flat "N concurrent uploads" limit has to be sized for the worst case (a 500 MB video), which
+//! makes it absurdly restrictive for the common case (a 3 MB photo). Budgeting bytes lets one
+//! 500 MB video and two hundred photos coexist under the same ceiling, and it means the ceiling is
+//! stated in the unit the disk actually cares about.
+//!
+//! The reservation is the streaming CAP, not the real size — the real size is unknowable until the
+//! body has been read, which is far too late. Reserving the cap is deliberately pessimistic; that
+//! pessimism is the safety margin.
+//!
+//! ## Why a permit and not a counter
+//!
+//! `OwnedSemaphorePermit` releases on drop. Every path out of the upload handler — success, error,
+//! a client vanishing mid-body, a panic — therefore returns the reservation without any explicit
+//! bookkeeping. A hand-rolled `AtomicI64` would need a decrement on each of those paths, and the
+//! one that gets missed is the one that leaks the budget until restart.
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use tokio::sync::{OwnedSemaphorePermit, Semaphore};
+
+/// Total transient upload bytes allowed on disk at once, in MiB.
+///
+/// Sized against `DISK_RESERVE_BYTES` (10 GB): the reserve must survive a full burst with room to
+/// spare, since Postgres is writing WAL to the same filesystem throughout. 4 GiB leaves ~6 GB of
+/// the reserve untouched at the worst moment.
+///
+/// It is NOT a throughput limit. On 2 vCPU the box cannot usefully ingest more than this at once
+/// anyway — compression, ffmpeg, Postgres and TLS all contend for the same two cores — so the
+/// budget mostly converts "everything is slow and the disk fills" into "a few uploads wait".
+const BUDGET_MIB: u32 = 4096;
+
+/// How long an upload waits for room before being told to come back.
+///
+/// Long enough to absorb the burst (a photo holds its reservation for well under a second), short
+/// enough that a guest is not left staring at a spinner. On timeout the handler answers 503 with
+/// `Retry-After`, which the client queue already treats as transient and retries with backoff.
+const WAIT: Duration = Duration::from_secs(20);
+
+#[derive(Clone)]
+pub struct UploadAdmission {
+    permits: Arc<Semaphore>,
+}
+
+impl UploadAdmission {
+    pub fn new() -> Self {
+        Self {
+            permits: Arc::new(Semaphore::new(BUDGET_MIB as usize)),
+        }
+    }
+
+    /// Reserve room for a body capped at `cap_bytes`. The returned permit must be held for as long
+    /// as the temp file exists.
+    ///
+    /// `None` means the wait timed out and the caller should shed the request.
+    ///
+    /// A cap larger than the whole budget is clamped rather than refused. Otherwise an operator
+    /// raising `max_video_size_mb` above the budget would make `acquire_many` unsatisfiable and
+    /// every video upload would hang until timeout — a config change silently disabling video for
+    /// the event. Clamped, such an upload simply gets the whole budget to itself, which is the
+    /// honest interpretation of "one file may fill the machine".
+    pub async fn reserve(&self, cap_bytes: usize) -> Option<OwnedSemaphorePermit> {
+        let mib = cap_bytes.div_ceil(1024 * 1024).max(1);
+        let want = u32::try_from(mib).unwrap_or(BUDGET_MIB).min(BUDGET_MIB);
+        match tokio::time::timeout(
+            WAIT,
+            self.permits.clone().acquire_many_owned(want),
+        )
+        .await
+        {
+            Ok(Ok(permit)) => Some(permit),
+            // The semaphore is never closed, so `Err` here is unreachable in practice; treat it
+            // the same as a timeout rather than panicking on the upload path.
+            Ok(Err(_)) => None,
+            Err(_) => {
+                tracing::warn!(
+                    requested_mib = want,
+                    "upload admission timed out; shedding to keep transient temp files bounded"
+                );
+                None
+            }
+        }
+    }
+}
+
+impl Default for UploadAdmission {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The budget must actually block once exhausted — otherwise this whole module is decoration.
+    #[tokio::test]
+    async fn a_full_budget_sheds_instead_of_admitting() {
+        let admission = UploadAdmission::new();
+        let whole = admission
+            .reserve(BUDGET_MIB as usize * 1024 * 1024)
+            .await
+            .expect("first reservation takes the whole budget");
+
+        // Nothing left: a second reservation must not be granted. Raced against a short timeout so
+        // the test does not sit for the full WAIT.
+        let blocked = tokio::time::timeout(
+            Duration::from_millis(150),
+            admission.reserve(1024 * 1024),
+        )
+        .await;
+        assert!(blocked.is_err(), "budget exhausted, yet a reservation was granted");
+
+        // ...and releasing the permit makes room again, so the budget is not a one-way latch.
+        drop(whole);
+        assert!(
+            admission.reserve(1024 * 1024).await.is_some(),
+            "budget did not recover after the permit was dropped"
+        );
+    }
+
+    /// A cap above the whole budget must be clamped, not left unsatisfiable. Unclamped,
+    /// `acquire_many` for more permits than exist never completes, so raising
+    /// `max_video_size_mb` past the budget would silently hang every video upload for 20s and
+    /// then shed it.
+    #[tokio::test]
+    async fn a_cap_larger_than_the_budget_is_clamped_rather_than_unsatisfiable() {
+        let admission = UploadAdmission::new();
+        let oversized = (BUDGET_MIB as usize + 4096) * 1024 * 1024;
+        assert!(
+            admission.reserve(oversized).await.is_some(),
+            "an over-budget cap must still be admittable on an idle server"
+        );
+    }
+}