Gateway registration model

The backend now has the database shape needed for account-owned gateway registration and client discovery.

PocketWorkers extends the account model with gateway records.

The gateway table tracks ownership, identity, display information, status, and route metadata. This gives the web app and clients a shared place to discover already-connected gateways.

The next acceptance step is to finish the authenticated AgentGateway registration loop and verify that clients can select a gateway without entering network details.