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/sse.rs

@@ -6,6 +6,7 @@ use axum::response::sse::{Event, KeepAlive, Sse};
 use axum::Json;
 use futures::stream::Stream;
 use serde::{Deserialize, Serialize};
+use tokio_stream::wrappers::errors::BroadcastStreamRecvError;
 use tokio_stream::wrappers::BroadcastStream;
 use tokio_stream::StreamExt;
 
@@ -62,7 +63,14 @@ pub async fn stream(
         Ok(sse_event) => Some(Ok(Event::default()
             .event(sse_event.event_type)
             .data(sse_event.data))),
-        Err(_) => None,
+        // A consumer that falls behind the broadcast buffer would otherwise silently
+        // lose events, leaving its feed permanently stale. Instead of dropping the
+        // gap, tell the client to resync — the frontend responds by running a full
+        // feed-delta fetch (which reconciles both new uploads and deletions).
+        Err(BroadcastStreamRecvError::Lagged(n)) => {
+            tracing::warn!("SSE consumer lagged, dropped {n} event(s); emitting resync");
+            Some(Ok(Event::default().event("resync").data(n.to_string())))
+        }
     });
 
     Ok(Sse::new(stream).keep_alive(