feat: in-process crawler daemon with cron and worker pool (0.28.0)

The backend now boots an internal crawler daemon that runs a daily
metadata pass (CRAWLER_DAILY_AT in CRAWLER_TZ, advisory-lock guarded
for multi-replica safety) and drains SyncChapterContent jobs from
crawler_jobs through a worker pool. Chromium launches lazily on first
job and is torn down after CRAWLER_IDLE_TIMEOUT_S seconds of inactivity.

Modules:
- crawler::browser_manager — lazy-launch / idle-teardown wrapper
  around browser::Handle, with an on_launch hook that re-injects
  PHPSESSID on every fresh Chromium spawn.
- crawler::pipeline — run_metadata_pass (the shared discover/upsert
  /cover/sync-chapters loop) and the enqueue_bookmarked_pending helper
  used by the cron tick.
- crawler::daemon — cron task + worker pool, behind two trait seams
  (MetadataPass, ChapterDispatcher) so tests can inject stubs without
  standing up Chromium or a live source.

Behavior:
- CRAWLER_DAEMON=false skips daemon spawn entirely (default for tests).
- Catch-up tick fires on startup if the last persisted slot was missed.
- A SyncOutcome::SessionExpired sets a sticky AtomicBool; workers
  idle until operator restart with a refreshed PHPSESSID.
- Worker dispatch wrapped in catch_unwind so a panicking handler
  marks the job failed instead of taking down the worker.
- Migration 0015 adds a small crawler_state k-v table for the
  last_metadata_tick_at watermark.

Dep additions: chrono-tz (IANA TZ parsing).

CLI (bin/crawler) reuses pipeline::run_metadata_pass and now holds
the browser via BrowserManager so the on_launch session injection
flow stays in one place. Inline chapter-content sync semantics are
unchanged — the queue is for the daemon, force-refetches and manual
backfills still bypass it.

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.27.0"
+version = "0.28.0"
 edition = "2021"
 default-run = "mangalord"
 
@@ -23,6 +23,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 uuid = { version = "1", features = ["v4", "serde"] }
 chrono = { version = "0.4", features = ["serde"] }
+chrono-tz = "0.9"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tower = { version = "0.5", features = ["util"] }