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.lock

@@ -397,6 +397,28 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "chrono-tz"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93698b29de5e97ad0ae26447b344c482a7284c737d9ddc5f9e52b74a336671bb"
+dependencies = [
+ "chrono",
+ "chrono-tz-build",
+ "phf 0.11.3",
+]
+
+[[package]]
+name = "chrono-tz-build"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0c088aee841df9c3041febbb73934cfc39708749bf96dc827e3359cd39ef11b1"
+dependencies = [
+ "parse-zoneinfo",
+ "phf 0.11.3",
+ "phf_codegen 0.11.3",
+]
+
 [[package]]
 name = "concurrent-queue"
 version = "2.5.0"
@@ -1448,7 +1470,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.27.0"
+version = "0.28.0"
 dependencies = [
  "anyhow",
  "argon2",
@@ -1459,6 +1481,7 @@ dependencies = [
  "bytes",
  "chromiumoxide",
  "chrono",
+ "chrono-tz",
  "dotenvy",
  "futures-core",
  "futures-util",
@@ -1868,6 +1891,15 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "parse-zoneinfo"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1f2a05b18d44e2957b88f96ba460715e295bc1d7510468a2f3d3b44535d26c24"
+dependencies = [
+ "regex",
+]
+
 [[package]]
 name = "password-hash"
 version = "0.5.0"