Graceful shutdown for background tasks

[longzhi]

Problem

Background tasks spawned via tokio::spawn across gateway, channels, and server have no cancellation mechanism:

• Delivery listeners, approval listeners, channel bot loops all run indefinitely
• No CancellationToken or JoinHandle tracking
• Server restart may lose in-flight messages
• No way to know when all background tasks have completed during shutdown

Proposed Solution

1. Implement tokio_util::CancellationToken propagation from the top-level runtime down to all spawned tasks
2. Track JoinHandles for spawned tasks to await completion during shutdown
3. Add shutdown timeout — force-kill tasks that don't respond to cancellation within N seconds
4. Log in-flight work status during shutdown sequence

Impact

• Data safety — in-flight messages are processed before shutdown
• Clean restarts — no orphaned tasks or leaked resources
• Observability — shutdown logs show what was still running