feat(server): implement chat file TTL cleanup and enhance JPush vendor channel support

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.

internal/config/server.go

@@ -61,3 +61,11 @@ type UploadConfig struct {
 	MaxFileSize  int64    `mapstructure:"max_file_size"`
 	AllowedTypes []string `mapstructure:"allowed_types"`
 }
+
+// FileCleanupConfig 聊天文件 TTL 过期清理配置
+type FileCleanupConfig struct {
+	Enabled         bool `mapstructure:"enabled"`          // 是否启用清理 worker
+	RetentionDays   int  `mapstructure:"retention_days"`   // 保留天数，默认 7
+	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
+}