feat(sync): concurrent drains, atomic watermarks, graceful Ctrl+C shutdown

Three fixes to the sync pipeline:

1. Atomic watermarks: wrap complete_job + update_watermark in a single
   SQLite transaction so crash between them can't leave partial state.

2. Concurrent drain loops: prefetch HTTP requests via join_all (batch
   size = dependent_concurrency), then write serially to DB. Reduces
   ~9K sequential requests from ~19 min to ~2.4 min.

3. Graceful shutdown: install Ctrl+C handler via ShutdownSignal
   (Arc<AtomicBool>), thread through orchestrator/CLI, release locked
   jobs on interrupt, record sync_run as "failed".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/mod.rs

@@ -12,6 +12,7 @@ pub mod paths;
 pub mod payloads;
 pub mod project;
 pub mod references;
+pub mod shutdown;
 pub mod sync_run;
 pub mod time;
 pub mod timeline;