feat(scouts): register GmailConnector at startup

Adds GmailConnector registration to the FastAPI lifespan startup block,
making it available via the connector registry for the ScoutEngine
and any other startup-time consumers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/main.py

@@ -79,6 +79,11 @@ async def _memory_cron_tick() -> None:
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    # Startup: register source connectors.
+    from app.scouts.connectors.gmail import GmailConnector  # noqa: PLC0415
+    from app.scouts.connectors.registry import register_connector  # noqa: PLC0415
+    register_connector(GmailConnector())
+
     # Startup: ensure agent tool modules are loaded.
     import app.agents  # noqa: F401