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>

.env.example

@@ -52,6 +52,23 @@ AUTH_RATE_BURST=10
 # on different hosts. Example: https://app.example.com,https://app.example.de
 CORS_ALLOWED_ORIGINS=
 
+# ----- Admin CSRF allowlist -----
+# Browser origins (scheme + host[:port]) permitted to POST to
+# /api/v1/admin/* mutating endpoints. Defends the session-cookie-
+# authenticated admin surface against SameSite=Lax form-POST CSRF.
+# Same shape as CORS_ALLOWED_ORIGINS (comma-separated). Compare against
+# the request's Origin header (falling back to Referer when absent);
+# safe methods (GET/HEAD/OPTIONS) are not checked, and requests with
+# neither Origin nor Referer (curl, server-to-server callers) are
+# always allowed.
+#
+# Default is empty: CSRF check disabled (operator opt-out). For a
+# browser-exposed deployment this should be set to the SvelteKit
+# origin, e.g. https://app.example.com. For a same-origin
+# docker-compose deploy where only one origin exists, set the same
+# value the browser uses.
+ADMIN_ALLOWED_ORIGINS=
+
 # ----- Upload limits -----
 # Per-request body cap. axum rejects oversized requests with 413 before
 # our handlers run. Default 200 MiB.
@@ -78,6 +95,20 @@ CRAWLER_MAX_IMAGE_BYTES=33554432
 # and the `bin/crawler` CLI). 0 means no cap — let the source walker run
 # to completion. Useful for capped test runs against a new source.
 CRAWLER_LIMIT=0
+
+# ----- Crawler reliability knobs -----
+# Hard upper bound on a single chapter-content job dispatch (seconds).
+# A job that exceeds the budget is acked failed (with exponential
+# backoff) instead of wedging a worker. Default 600s.
+CRAWLER_JOB_TIMEOUT_SECS=600
+# Consecutive metadata-pass `fetch_manga` failures that abort the pass
+# (circuit breaker for a source outage). The pass does NOT mark a clean
+# exit, so the next tick does a recovery sweep. Default 10.
+CRAWLER_METADATA_MAX_CONSECUTIVE_FAILURES=10
+# Consecutive transient chapter failures (after TOR recircuit is
+# exhausted) that trigger an automatic coordinated browser restart.
+# Default 3.
+CRAWLER_BROWSER_RESTART_THRESHOLD=3
 # Path to a system Chromium binary. When set, the crawler skips the
 # bundled-fetcher download. Required on platforms without a usable
 # upstream Chromium build (notably Linux_arm64 / Raspberry Pi). On