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/src/main.rs

@@ -12,10 +12,21 @@ async fn main() -> anyhow::Result<()> {
 
     let config = mangalord::config::Config::from_env()?;
     let addr: SocketAddr = config.bind_address.parse()?;
-    let app = mangalord::app::build(config).await?;
+    let mangalord::app::AppHandle { router, daemon } = mangalord::app::build(config).await?;
 
     tracing::info!(%addr, "mangalord listening");
     let listener = tokio::net::TcpListener::bind(addr).await?;
-    axum::serve(listener, app).await?;
+    axum::serve(listener, router)
+        .with_graceful_shutdown(async {
+            let _ = tokio::signal::ctrl_c().await;
+            tracing::info!("ctrl-c received; shutting down");
+        })
+        .await?;
+
+    // Drain background tasks (crawler daemon) before exiting so Chromium
+    // gets a clean shutdown rather than relying on kill-on-drop.
+    if let Some(d) = daemon {
+        d.shutdown().await;
+    }
     Ok(())
 }