- Add api_key field to MonitoredServer model
- Add migration to create api_key column with unique index
- Add /admin/servers/{id}/api-key endpoint for key generation
- Add /admin/servers/{id}/api-key DELETE endpoint for revocation
- Add /server/heartbeat-v2 endpoint with X-API-Key header authentication
- Add TelemetryPayload model with extended fields (load_avg, uptime_seconds)
- Add basic unit tests for API key functionality
