PSv2: Service "live" status for async processing services

[carlosgjs]

Summary

In the asynchonous/pull model, antenna doesn't have an endpoint to call on the services to check for liveness. Instead, the service workers poll antenna for work to do. We need to update the status of such services to show as "live" based on the time since the last poll.

Related Issues

Depends on #987

[mihow]

Claude says: The core "live" status (last-poll heartbeat plus periodic offline marking) shipped in #1146 (merged). The remaining per-individual-service granularity is covered by the in-flight API-key identity work, PR #1194 / #1201. Linking for tracking; this can close once #1194 lands if the per-service view is considered in scope.