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

configs/config.yaml

@@ -159,6 +159,14 @@ upload:
     - image/gif
     - image/webp
 
+# 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
+# 环境变量: APP_DEVICE_CLEANUP_ENABLED, APP_DEVICE_CLEANUP_RETENTION_DAYS, APP_DEVICE_CLEANUP_INTERVAL_MINUTES
+device_cleanup:
+  enabled: true             # 是否启用清理 worker
+  retention_days: 3         # 保留天数，3 天不活跃即清理（按 last_used_at 判断）
+  interval_minutes: 360     # 扫描间隔（分钟），默认 6 小时
+
 # 敏感词过滤配置
 sensitive:
   enabled: true