Replace the minimal sync-only shutdown (which only saved autonomous state)
with a comprehensive async graceful_shutdown() coroutine that:
1. Ends active voice sessions (disconnect, release GPU locks, cleanup audio)
2. Saves autonomous engine state
3. Stops the APScheduler
4. Cancels all tracked background tasks (from task_tracker)
5. Closes the Discord gateway connection
Signal handlers (SIGTERM/SIGINT) now schedule the async shutdown on the
running event loop. The atexit handler is kept as a last-resort sync fallback.
Resolves#5, also addresses #4 (voice cleanup at shutdown)
koko210Serve
8d5137046c