Add fingerprint-based detection for duplicate device registration with
configurable behavior via new body params:
- allow_existing: return existing device if fingerprint matches
- allow_duplicate: skip fingerprint check, always create new device
- reset_syncs: clear tracked saves when reclaiming existing device
Fingerprint matching uses mac_address (primary) or hostname+platform
(fallback). Returns 409 Conflict with device_id when duplicate detected
without flags, 200 OK for existing device, 201 Created for new.
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
