Add presence heartbeat to keep bot showing as online

matrix-nio's AsyncClient.set_presence() is called every 60s in a
background task so Matrix clients see the bot as online. On shutdown
the bot sets itself to offline.

https://claude.ai/code/session_01DuzWyMMXvLMB4VxEwJyV4X

app/main.py

@@ -4,7 +4,7 @@ from contextlib import asynccontextmanager
 from fastapi import FastAPI
 
 from app.database import close_db, get_thread_root, init_db, set_thread_root
-from app.matrix import THREAD_ROOT_MESSAGES, close_client, send_thread_root
+from app.matrix import THREAD_ROOT_MESSAGES, close_client, send_thread_root, start_presence
 from app.webhooks import router as webhook_router
 
 logging.basicConfig(
@@ -19,6 +19,7 @@ async def lifespan(app: FastAPI):
     log.info("Starting Melora")
     await init_db()
     await _ensure_thread_roots()
+    await start_presence()
     log.info("Melora ready")
     yield
     log.info("Shutting down Melora")