feat(crawler): reliability fixes — heartbeat, streaming, jitter, timeout, breaker

A1 Lease heartbeat: jobs::renew keeps a long-but-healthy job's lease fresh
so it is never stolen mid-flight nor inflated toward max_attempts.
A2 Stream chapter pages straight to storage (peak memory = one image) and
persist rows + page_count in one short transaction off the network path
(S3-ready); roll back stored blobs on failure via Storage::delete.
A3 ±20% jitter on exponential backoff to avoid a retry thundering herd.
A4 Outer per-dispatch timeout (CRAWLER_JOB_TIMEOUT_SECS, default 600) so a
hung job is acked-failed instead of wedging a worker.
A5 Metadata circuit-breaker (CRAWLER_METADATA_MAX_CONSECUTIVE_FAILURES,
default 10): abort a pass on a source outage without marking a clean exit,
so the next tick recovery-sweeps.

Adds CRAWLER_BROWSER_RESTART_THRESHOLD config (used by the upcoming
coordinated browser restart). Bumps version 0.52.0 -> 0.53.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.52.0",
+  "version": "0.53.0",
   "private": true,
   "type": "module",
   "scripts": {