Adds three new monitoring endpoints:
- GET /health/gitea — Verifies Gitea API connectivity
- GET /health/discord — Reports Discord bot connection status
- GET /health/queue — Returns pending task count
All endpoints return JSON with status field. The Gitea endpoint
includes the API status code on success or error message on failure.
Discord endpoint returns "not_configured", "connecting", or "ok" with
bot username. Queue endpoint includes pending_tasks count.
Tests use mock lifespan to avoid initializing task queue, message
store, dispatcher, and discord handler during testing.
