Fix: non-blocking vault sync on startup

app.py

@@ -91,9 +91,10 @@ async def _periodic_vault_sync():
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    # Startup: sync vault, fall back to disk cache
-    if not _sync_vault():
+    # Startup: try vault sync in background, use file fallback immediately
     _load_disk_cache()
+    loop = asyncio.get_event_loop()
+    loop.run_in_executor(None, _sync_vault)  # non-blocking
     task = asyncio.create_task(_periodic_vault_sync())
     yield
     task.cancel()