Add DeviceCleanupWorker to periodically clean up stale device tokens that haven't
been used within the retention period. This prevents accumulation of invalid
registration_ids and maintains data hygiene.
- New DeviceCleanupConfig with enabled, retention_days (3), and interval_minutes (360)
- New repository methods DeleteStaleDevices and DeleteOldestActiveDevice
- Updated RegisterDevice to evict oldest active device when reaching limit
- Reduced MaxDevicesPerUser from 10 to 3 per user
- Worker integrated into app startup/shutdown lifecycle
- Migrate all log.Printf/log.Println calls to structured zap logging
- Use cmp.Or for cleaner default value handling
- Replace manual loops with slices.ContainsFunc/IndexFunc
- Add batch query methods (IsLikedBatch, IsFavoritedBatch) to solve N+1 problem
- Update JSON tags from omitempty to omitzero for numeric fields
- Use errgroup-style wg.Go for worker goroutines
- Remove duplicate casbin/v2 dependency (keeping v3)
- Add plans/ to gitignore
