feat(admin): crawler observability dashboard + reliability hardening (0.55.0)

Admin-only crawler dashboard backed by an SSE live-status stream,
coordinated browser restart, runtime PHPSESSID refresh, dead-letter
requeue, and a batch of reliability fixes. Closes everything from
the two-pass audit (10 commits' worth) and bumps 0.52.0 -> 0.55.0.

Backend:

- New /admin/crawler/* surface (cookie-auth, RequireAdmin) split
  into status / control / dead_jobs / backlog modules. SSE stream
  composes in-memory status with DB-derived queue counts, memoizes
  the counts for 1s and debounces watch pokes for 250ms (~10x QPS
  reduction per subscriber). One-shot GET /admin/crawler shares the
  same compose path.
- POST /admin/crawler/run gated by manual_pass_lock try_lock_owned
  (409 Conflict on overlapping click); browser restart goes through
  the coordinated_restart gate (drain + relaunch + auto-clear of the
  sticky session_expired flag on Ok).
- Runtime PHPSESSID refresh via SessionController (allow-list
  validation, never logged, audit row carries SHA-256 fingerprint).
  Storage layer is repo::crawler::runtime_session_{load,persist}.
- Dead-letter requeue with four scopes (all/manga/chapter/job);
  scope=all requires confirm:true; DISTINCT ON dedup keeps the
  partial unique index from rejecting requeues for chapters with
  multiple dead rows. SQL is four &'static str constants per scope.
- StatusHandle + ChapterGuard / CoverGuard RAII model survives
  panics; last-writer-wins on cover so concurrent dispatches don't
  clobber each other's slot. Pure functions (should_stop /
  should_mark_clean_exit / should_abort_pass) with named regression
  tests.
- Reliability bundle: per-lease heartbeat, jitter on retries,
  per-job timeout, circuit breaker on consecutive failures, BrowserManager
  coordinated restart gate, request fingerprint changes.
- Streaming page download: Storage::put_stream trait method,
  LocalStorage impl atomic via temp + fsync + UUID-suffixed rename.
  Pages stream through with peak memory ~one HTTP chunk + 64-byte
  sniff prefix instead of one full image per dispatch.
- New partial indexes (migration 0022): mangas_missing_cover_idx
  and crawler_jobs_dead_idx, both ordered by updated_at DESC to
  match the dashboard's LIMIT/OFFSET reads.
- Security hardening: admin_csrf_guard (Origin/Referer allowlist
  on /admin/* mutations, opt-in via ADMIN_ALLOWED_ORIGINS),
  admin_no_store_guard (Cache-Control: no-store on admin
  responses), audit rows carry per-scope target_id.

Frontend:

- /admin/crawler page decomposed into lib/components/crawler/
  (11 components: ProgressBar, SearchBar, CrawlerHero,
  CrawlerControls, ActiveChaptersCard, ActiveJobsTable,
  MissingCoversTable, DeadJobsTable, RestartConfirmModal,
  RequeueAllConfirmModal, SessionModal). Page is 532 LOC of
  orchestration; each component 22-148 LOC.
- EventSource lifecycle wired to visibilitychange / pagehide /
  pageshow (BFCache); after 5 consecutive errors probes the status
  endpoint so a 401 routes through the global on401Hook instead of
  infinite silent reconnects.
- Backlog $effect refetches debounced 500ms with per-loader
  AbortControllers; refresh after a control action only runs when
  the SSE stream is dead.
- Inline requeue button on /admin/mangas patches the affected row's
  sync_state locally (no full chapter-list refetch); proper
  aria-label. Requeue-all gets its own confirm modal; both confirm
  modals autofocus Cancel.
- SvelteKit reverse proxy bypasses its 5-minute AbortController
  for Accept: text/event-stream; pure shouldBypassProxyTimeout
  helper covered by unit tests.

Config / docs:

- New env vars (.env.example): ADMIN_ALLOWED_ORIGINS,
  CRAWLER_JOB_TIMEOUT_SECS, CRAWLER_METADATA_MAX_CONSECUTIVE_FAILURES,
  CRAWLER_BROWSER_RESTART_THRESHOLD.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/crawler_daemon.rs

@@ -40,6 +40,8 @@ fn make_cfg(
         tz: Tz::UTC,
         retention_days: 7,
         session_expired,
+        status: mangalord::crawler::status::StatusHandle::new(workers),
+        job_timeout: Duration::from_secs(60),
         extra_tasks: Vec::new(),
     }
 }
@@ -88,6 +90,52 @@ impl ChapterDispatcher for PanickingDispatcher {
     }
 }
 
+/// Never completes — used to verify the worker's outer dispatch timeout.
+struct HangingDispatcher {
+    seen: AtomicUsize,
+}
+#[async_trait::async_trait]
+impl ChapterDispatcher for HangingDispatcher {
+    async fn dispatch(&self, _payload: JobPayload) -> anyhow::Result<SyncOutcome> {
+        self.seen.fetch_add(1, Ordering::AcqRel);
+        std::future::pending::<()>().await;
+        unreachable!("hanging dispatcher never resolves");
+    }
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn worker_times_out_a_hung_dispatch_and_acks_failed(pool: PgPool) {
+    enqueue_chapter_job(&pool).await;
+    let dispatcher = Arc::new(HangingDispatcher {
+        seen: AtomicUsize::new(0),
+    });
+    let session_expired = Arc::new(std::sync::atomic::AtomicBool::new(false));
+    let cancel = CancellationToken::new();
+    let mut cfg = make_cfg(None, dispatcher.clone(), session_expired, 1);
+    cfg.job_timeout = Duration::from_millis(300);
+    let handle = daemon::spawn(pool.clone(), cancel.clone(), cfg);
+
+    // The hung job should time out and return to pending with backoff
+    // (attempts=1 < max=5). Poll for the recorded error.
+    let mut timed_out = false;
+    for _ in 0..40 {
+        let n: i64 = sqlx::query_scalar(
+            "SELECT COUNT(*) FROM crawler_jobs WHERE last_error = 'dispatch timed out'",
+        )
+        .fetch_one(&pool)
+        .await
+        .unwrap();
+        if n == 1 {
+            timed_out = true;
+            break;
+        }
+        tokio::time::sleep(Duration::from_millis(50)).await;
+    }
+    handle.shutdown().await;
+    assert!(timed_out, "hung dispatch must be acked failed with a timeout error");
+    assert!(dispatcher.seen.load(Ordering::Acquire) >= 1);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn workers_drain_jobs_through_dispatcher(pool: PgPool) {
     enqueue_chapter_job(&pool).await;