feat(trade): implement second-hand trading/flea market feature

Add complete trade functionality including:
- TradeItem, TradeImage, TradeFavorite models with auto-migration
- TradeRepository with CRUD and favorite operations
- TradeService with business logic for listing, creating, updating, status management
- TradeHandler with RESTful endpoints for trade operations
- DTOs and converters for request/response handling

Routes include: list, get by id, create, update, delete, status update, view recording, favorite/unfavorite.

internal/model/init.go

@@ -179,6 +179,11 @@ func autoMigrate(db *gorm.DB) error {
 
 		// 帖子内链引用关系
 		&PostReference{},
+
+		// 二手交易相关
+		&TradeItem{},
+		&TradeImage{},
+		&TradeFavorite{},
 	)
 	if err != nil {
 		return err