Replace internal goroutine-based timing loops with scheduler integration
for SyncOutboxService and ModelRegistryService. Both services now expose
RunOnce() methods called by the central scheduler instead of managing
their own background loops.
- Add Interval() and RunOnce() methods to SyncOutboxService
- Add RefreshEvery() and RunOnce() methods to ModelRegistryService
- Remove started flag from SyncOutboxService struct
- Move scheduler.Start() after all services are initialized
- Ensure initial model registry refresh before scheduler starts
- Introduce `SyncOutboxService` and model to retry failed CP-to-Redis sync operations
- Update `SyncService` to handle sync failures by enqueuing tasks to the outbox
- Centralize provider group and API key validation logic into `ProviderGroupManager`
- Refactor API handlers to utilize the new manager and robust sync methods
- Add configuration options for sync outbox (interval, batch size, retries)
