Adds asynccontextmanager lifespan to webapp.py that:
- Initializes task_queue and message_store on startup
- Starts Dispatcher as background task
- Starts Discord bot gateway if DISCORD_TOKEN is set
- Properly shuts down all resources on application exit
This completes Phase 3 Task 6, enabling the webapp to run Discord bot
and dispatcher concurrently with the webhook server.
- Add parse_gitea_event() function to parse issue comments, labels, and PR review requests
- Detect @agent mentions in issue comments and strip them from message
- Implement rate limiting with slowapi (10 requests/minute)
- Integrate with PersistentTaskQueue and MessageStore
- Queue messages if task is already running for the same thread
- Add 6 comprehensive tests for event parsing and signature verification
