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>

Cargo.toml

@@ -30,7 +30,7 @@ open = "5"
 
 # HTTP
 reqwest = { version = "0.12", features = ["json"] }
-tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time", "signal"] }
 
 # Async streaming for pagination
 async-stream = "0.3"