Move the vendor switch logic into the ThirdPartyChannel struct as Set/IsEmpty/Normalize
methods to improve encapsulation and eliminate code duplication in the vendor params
builder. Also simplify messageToParams to reuse buildMessagePayload results and remove
unused notification_type extras from push notifications.
Add FileCleanupWorker for automatic expiration of chat files with configurable retention period and batch processing. Files uploaded via `/api/v1/uploads/files` are tracked in `uploaded_files` table with expiration timestamps, then deleted from S3 and database upon expiry. Expired file URLs are injected as `"expired": true` in message responses.
Extend JPush push notification configuration with vendor-specific channel_id support (xiaomi, huawei, oppo, vivo, meizu, honor, fcm) for differentiating system vs chat notifications. Add iOS APNs thread-id grouping configuration for notification categorization.
