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/repository/device_token_repo.go

@@ -31,6 +31,12 @@ type DeviceTokenRepository interface {
 	GetDeviceCountByUserID(userID int64) (int64, error)
 	GetActiveDeviceCountByUserID(userID int64) (int64, error)
 	DeleteInactiveDevices(before time.Time) error
+	// DeleteStaleDevices 删除超过指定时间未使用的设备（按 last_used_at 判断，不限 is_active）
+	// 用于周期性清理长期不活跃的 registration_id，last_used_at 为 NULL 的设备不清理
+	DeleteStaleDevices(before time.Time) error
+	// TrimActiveDevices 将用户活跃设备数量收敛到 keep 以内，删除超额的最旧设备
+	// 返回实际删除的记录数。用于设备数限制（如限制从 10 改为 3 后的存量收敛）
+	TrimActiveDevices(userID string, keep int) (int64, error)
 }
 
 // deviceTokenRepository 设备Token仓储实现
@@ -200,3 +206,31 @@ func (r *deviceTokenRepository) DeleteInactiveDevices(before time.Time) error {
 	return r.db.Where("is_active = ? AND last_used_at < ?", false, before).
 		Delete(&model.DeviceToken{}).Error
 }
+
+// DeleteStaleDevices 删除超过指定时间未使用的设备（按 last_used_at 判断，不限 is_active）
+// last_used_at 为 NULL 的设备不清理（可能是刚注册尚未使用）
+func (r *deviceTokenRepository) DeleteStaleDevices(before time.Time) error {
+	return r.db.Where("last_used_at IS NOT NULL AND last_used_at < ?", before).
+		Delete(&model.DeviceToken{}).Error
+}
+
+// TrimActiveDevices 将用户活跃设备数量收敛到 keep 以内，删除超额的最旧设备
+// 按 last_used_at 降序保留最新的 keep 条活跃记录，删除其余活跃记录。
+// keep <= 0 时不删除任何记录。返回实际删除的记录数。
+// 用于设备数限制（新注册超限淘汰 + 历史存量超额收敛）。
+func (r *deviceTokenRepository) TrimActiveDevices(userID string, keep int) (int64, error) {
+	if keep <= 0 {
+		return 0, nil
+	}
+	// 子查询：取该用户最新的 keep 条活跃记录 ID
+	subQuery := r.db.Model(&model.DeviceToken{}).
+		Select("id").
+		Where("user_id = ? AND is_active = ?", userID, true).
+		Order("last_used_at DESC NULLS LAST").
+		Limit(keep)
+
+	// 删除不在保留集合内的活跃记录
+	result := r.db.Where("user_id = ? AND is_active = ? AND id NOT IN (?)", userID, true, subQuery).
+		Delete(&model.DeviceToken{})
+	return result.RowsAffected, result.Error
+}