perf(backend): close config/upload/export hot paths + stability fixes

Performance:
- Cache the runtime `config` table in-memory (ConfigCache) with synchronous
  invalidation on every write (admin PATCH + test reseed). Was re-reading each
  key from Postgres on every request (~8 round-trips per upload).
- Stream uploads chunk-by-chunk to a temp file instead of buffering the whole
  body in RAM (peak was up to the per-class cap, e.g. 500 MB/video); only 512
  sniff-bytes are kept for magic-byte detection, then atomic rename into place.
- Cache the media-filesystem disk snapshot (DiskCache, 15s TTL) shared by the
  quota check and admin stats; drop the discarded System::refresh_all().
- HTML export streams video (and small-image) originals straight into the ZIP
  via a manifest instead of copying them to a temp dir first (removed the
  transient 2x disk usage) and drops the double directory scan.
- Auth extractor resolves session -> live user in one JOIN (was two queries),
  touching last_seen_at by token hash.

Stability:
- SSE: on broadcast lag, emit a `resync` event so the client runs a delta
  fetch instead of silently losing events; frontend reconciles adds, deletions,
  and (via an in-place refresh) like/comment counts on visible cards.
- Storage quota fails OPEN when the disk can't be read (was a 0-byte limit that
  locked out all uploads).
- Graceful shutdown drains in-flight requests on SIGTERM/SIGINT, bounded by a
  10s backstop so open SSE streams can't stall a deploy.
- Upload removes the persisted file if the DB transaction fails (no orphaned
  bytes with no row to reclaim them).

Tests:
- New pure select_disk() with 5 unit tests (longest-prefix, fallbacks, fail-open).
- New e2e export-video spec covering the HTML export's video-streaming branch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/handlers/admin.rs

@@ -5,7 +5,6 @@ use axum::extract::{Query, State};
 use axum::http::{HeaderMap, StatusCode};
 use axum::Json;
 use serde::{Deserialize, Serialize};
-use sysinfo::System;
 
 use crate::auth::middleware::RequireAdmin;
 use crate::error::AppError;
@@ -68,23 +67,12 @@ pub async fn get_stats(
     .fetch_one(&state.pool)
     .await?;
 
-    // Disk usage via sysinfo
-    let mut sys = System::new();
-    sys.refresh_all();
-
-    let media_path = state.config.media_path.to_string_lossy().to_string();
-    let (disk_total, disk_free) = sysinfo::Disks::new_with_refreshed_list()
-        .iter()
-        .find(|d| media_path.starts_with(d.mount_point().to_string_lossy().as_ref()))
-        .map(|d| (d.total_space(), d.available_space()))
-        .unwrap_or_else(|| {
-            // Fall back to the root disk
-            sysinfo::Disks::new_with_refreshed_list()
-                .iter()
-                .find(|d| d.mount_point().to_string_lossy() == "/")
-                .map(|d| (d.total_space(), d.available_space()))
-                .unwrap_or((0, 0))
-        });
+    // Disk usage from the shared cache (unknown mount → zeros, same as before).
+    let (disk_total, disk_free) = state
+        .disk_cache
+        .snapshot(&state.config.media_path)
+        .map(|d| (d.total, d.free))
+        .unwrap_or((0, 0));
 
     let disk_used = disk_total.saturating_sub(disk_free);
 
@@ -216,6 +204,11 @@ pub async fn patch_config(
     }
     tx.commit().await?;
 
+    // The config cache must reflect this write on the very next read (tests PATCH then
+    // immediately assert the new value takes effect). Invalidate synchronously here —
+    // the TTL is only a backstop and must not be relied on for correctness.
+    state.config_cache.invalidate();
+
     // Notify all clients that a publicly-readable config value changed so their stores
     // (e.g. the privacy note in My Account) refresh without a manual reload.
     if privacy_note_changed {
@@ -408,13 +401,13 @@ pub async fn export_status(
 /// switch + per-endpoint switch + numeric value, all stored in `config` and read on
 /// each request.
 async fn enforce_export_rate(state: &AppState, headers: &HeaderMap) -> Result<(), AppError> {
-    let rate_limits_on = config::get_bool(&state.pool, "rate_limits_enabled", true).await;
-    let export_rate_on = config::get_bool(&state.pool, "export_rate_enabled", true).await;
+    let rate_limits_on = config::get_bool(&state.config_cache, "rate_limits_enabled", true).await;
+    let export_rate_on = config::get_bool(&state.config_cache, "export_rate_enabled", true).await;
     if !(rate_limits_on && export_rate_on) {
         return Ok(());
     }
     let ip = client_ip(headers, "unknown");
-    let limit = config::get_usize(&state.pool, "export_rate_per_day", 3).await;
+    let limit = config::get_usize(&state.config_cache, "export_rate_per_day", 3).await;
     if !state
         .rate_limiter
         .check(format!("export:{ip}"), limit, Duration::from_secs(86400))