Replace lifespan with on_event handlers for older FastAPI compat

The lifespan constructor parameter was added in FastAPI 0.93. When
running outside Docker with an older system-packaged FastAPI, the
parameter is silently ignored — the app starts and serves routes but
init_db() and start_presence() never run, so the bot never goes online
and webhooks crash with "Database not initialised".

on_event("startup")/on_event("shutdown") work across all FastAPI
versions.

https://claude.ai/code/session_019ANRdyL2jfi7ysWhqN4PfD

app/main.py

@@ -1,5 +1,4 @@
 import logging
-from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 
@@ -13,18 +12,8 @@ logging.basicConfig(
 )
 log = logging.getLogger(__name__)
 
-
-@asynccontextmanager
-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")
-    await close_client()
-    await close_db()
+app = FastAPI(title="Melora")
+app.include_router(webhook_router)
 
 
 async def _ensure_thread_roots() -> None:
@@ -38,8 +27,20 @@ async def _ensure_thread_roots() -> None:
         await set_thread_root(media_type, event_id)
 
 
-app = FastAPI(title="Melora", lifespan=lifespan)
-app.include_router(webhook_router)
+@app.on_event("startup")
+async def startup() -> None:
+    log.info("Starting Melora")
+    await init_db()
+    await _ensure_thread_roots()
+    await start_presence()
+    log.info("Melora ready")
+
+
+@app.on_event("shutdown")
+async def shutdown() -> None:
+    log.info("Shutting down Melora")
+    await close_client()
+    await close_db()
 
 
 @app.get("/health")