feat: crawler job queue ops and dedup index (0.27.0)

Adds enqueue / lease / ack_done / ack_failed / release / reap_done on
crawler::jobs, backed by the existing crawler_jobs table. lease() uses
a single FOR UPDATE SKIP LOCKED CTE that also re-claims stale running
rows (crashed-worker recovery), and ack_failed applies an exponential
backoff capped at 1h before retrying.

Migration 0014 adds a partial unique index on
(payload->>'chapter_id') restricted to (pending|running)
sync_chapter_content jobs, so producers can just
INSERT ... ON CONFLICT DO NOTHING without racing each other. The slot
frees again the moment the job leaves the in-flight states, so a
future force-refetch can re-enqueue.

Library-only — no daemon, no API hook. Those land in the next two
phases.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.26.1",
+  "version": "0.27.0",
   "private": true,
   "type": "module",
   "scripts": {