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/src/crawler/safety.rs

@@ -241,6 +241,31 @@ pub async fn fetch_bytes_capped(
         .with_context(|| format!("download body for {url}"))
 }
 
+/// Send `req` and return the response body as a stream after the
+/// safety check + 2xx status check. Caller owns chunking, capping, and
+/// piping to storage. Used by `download_and_store_page` so peak memory
+/// stays at one chunk per concurrent dispatch instead of one full
+/// image.
+pub async fn fetch_stream(
+    http: &reqwest::Client,
+    url: &str,
+    referer: Option<&str>,
+    allow: &DownloadAllowlist,
+) -> anyhow::Result<reqwest::Response> {
+    is_safe_url(url, allow).with_context(|| format!("reject unsafe URL {url}"))?;
+    let mut req = http.get(url);
+    if let Some(r) = referer {
+        req = req.header(reqwest::header::REFERER, r);
+    }
+    let resp = req
+        .send()
+        .await
+        .with_context(|| format!("GET {url}"))?
+        .error_for_status()
+        .with_context(|| format!("non-2xx for {url}"))?;
+    Ok(resp)
+}
+
 /// True when `bytes` sniffs as one of the *renderable* image formats
 /// the `/files/*key` endpoint can serve with a correct Content-Type:
 /// JPEG, PNG, WebP, GIF, AVIF. Matches the upload pipeline's