Implement device registration and save sync tracking to enable
multi-device save management with conflict detection.
- Device CRUD endpoints (POST/GET/PUT/DELETE /api/devices)
- Save sync state tracking per device
- Conflict detection on upload (409 when device has stale sync)
- Download sync tracking (optimistic and confirmed modes)
- Track/untrack saves per device
- DEVICES_READ/WRITE scopes for authorization
