#4365 serve: define what swamp serve does when its OAuth client is deleted at swamp-club

Context: swamp-club#4263 adds deleting a collective's OAuth clients from the API and the serve page. Deleting a client that is in use (force) removes its serve_instances row, so that instance's next club heartbeat gets a 404 and new device logins through the client fail. Sessions already issued keep working.

Today swamp serve does not handle this: ClubHeartbeatService (src/serve/club_heartbeat_service.ts) logs Club heartbeat failed as a warning on every interval and keeps running, with nothing that tells the operator the client is gone or what to do.

Request: decide and implement serve's behaviour when its client is deleted. Options: log one clear error that names the deleted client and the recovery steps (re-register, or supply --oauth-client-id with its secret), stop the heartbeat loop, surface it in serve status, or exit so the orchestrator restarts the instance.

Split out of swamp-club#4264 at the maintainer's request; related to swamp-club#4263.