fix(user-flow): close offline-queue, export, session, ban & realtime gaps

Comprehensive user-flow review across guest/host/admin roles, then fixes with
e2e regression guards. Highlights:

- Offline upload queue auto-resumes on reconnect: network errors keep items
  pending (not error), 4xx are terminal (no infinite retry), quota exhaustion
  returns a distinct 413; queue cap + dedup.
- Export lifecycle: release atomically locks uploads; reopen invalidates and
  re-release regenerates the keepsake; workers claim their job atomically so a
  reopen->re-release can't corrupt the ZIP; startup re-spawns interrupted
  exports.
- Sessions slide on activity (no 30-day cliff); JWT expiry deferred to the
  revocable session row; sign-out-everywhere + revoke-on-PIN-reset.
- Ban always hides content (v_feed / find_visible_media / export filter
  is_banned) but stays a read-only ban per USER_JOURNEYS §10 — sessions are
  not revoked, read access + keepsake download preserved.
- Realtime: server-clock SSE delta cursor; event-closed/opened drive the UI
  live; feed_delta rate-limited; like returns {liked, like_count} to fix
  multi-device drift; lightbox live comments; diashow delta backfill.
- Forgotten-PIN in-app request flow; simultaneous same-name join returns 409;
  quota increment is transactional; operator floor.

Adds e2e/specs/10-flow-review/ (offline resume, export integrity, deterministic
anti-race guard) and updates existing specs for the new contracts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/state.rs

@@ -42,7 +42,11 @@ pub struct AppState {
 
 impl AppState {
     pub fn new(pool: PgPool, config: AppConfig) -> Self {
-        let (sse_tx, _) = broadcast::channel(256);
+        // Broadcast buffer for live SSE fan-out. Sized to absorb a burst (e.g. many
+        // uploads landing at once during a busy moment) before a slow consumer lags and
+        // has to `resync`. The resync path is a correctness backstop, not the happy path —
+        // a roomier buffer keeps ~1000 concurrent clients from all resyncing at once.
+        let (sse_tx, _) = broadcast::channel(1024);
         let compression = CompressionWorker::new(
             pool.clone(),
             config.media_path.clone(),