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/feed.rs

@@ -62,10 +62,10 @@ pub async fn feed(
     Query(q): Query<FeedQuery>,
 ) -> Result<Json<FeedResponse>, AppError> {
     let ip = client_ip(&headers, "unknown");
-    let rate_limits_on = config::get_bool(&state.pool, "rate_limits_enabled", true).await;
-    let feed_rate_on = config::get_bool(&state.pool, "feed_rate_enabled", true).await;
+    let rate_limits_on = config::get_bool(&state.config_cache, "rate_limits_enabled", true).await;
+    let feed_rate_on = config::get_bool(&state.config_cache, "feed_rate_enabled", true).await;
     if rate_limits_on && feed_rate_on {
-        let rate_limit = config::get_usize(&state.pool, "feed_rate_per_min", 60).await;
+        let rate_limit = config::get_usize(&state.config_cache, "feed_rate_per_min", 60).await;
         if !state
             .rate_limiter
             .check(format!("feed:{ip}"), rate_limit, Duration::from_secs(60))