feat(worker): add device registration_id cleanup worker

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

internal/config/server.go

@@ -69,3 +69,11 @@ type FileCleanupConfig struct {
 	IntervalMinutes int  `mapstructure:"interval_minutes"` // 扫描间隔（分钟），默认 360（6 小时）
 	BatchSize       int  `mapstructure:"batch_size"`       // 单次扫描处理上限，默认 100
 }
+
+// DeviceCleanupConfig 设备 registration_id 清理配置
+// 周期性清理超过保留期未使用的设备 token，避免无效 registration_id 累积
+type DeviceCleanupConfig struct {
+	Enabled         bool `mapstructure:"enabled"`          // 是否启用清理 worker，默认 true
+	RetentionDays   int  `mapstructure:"retention_days"`   // 保留天数，默认 3（3 天不活跃即清理）
+	IntervalMinutes int  `mapstructure:"interval_minutes"` // 扫描间隔（分钟），默认 360（6 小时）
+}