refactor: update gateway agent session key handling and improve related logic

backend/scripts/seed_demo.py

@@ -18,6 +18,7 @@ async def run() -> None:
     from app.models.boards import Board
     from app.models.gateways import Gateway
     from app.models.users import User
+    from app.services.gateway_agents import gateway_agent_session_key
 
     await init_db()
     async with async_session_maker() as session:
@@ -26,9 +27,10 @@ async def run() -> None:
             name="Demo Gateway",
             url="http://localhost:8080",
             token=None,
-            main_session_key="demo:main",
+            main_session_key="placeholder",
             workspace_root=str(demo_workspace_root),
         )
+        gateway.main_session_key = gateway_agent_session_key(gateway)
         session.add(gateway)
         await session.commit()
         await session.refresh(gateway)