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/dto/trade_converter.go

@@ -0,0 +1,101 @@
+package dto
+
+import (
+	"with_you/internal/model"
+)
+
+func ConvertTradeImageToResponse(img *model.TradeImage) TradeImageResponse {
+	if img == nil {
+		return TradeImageResponse{}
+	}
+	return TradeImageResponse{
+		ID:              img.ID,
+		URL:             img.URL,
+		ThumbnailURL:    img.ThumbnailURL,
+		PreviewURL:      img.PreviewURL,
+		PreviewURLLarge: img.PreviewURLLarge,
+		Width:           img.Width,
+		Height:          img.Height,
+	}
+}
+
+func ConvertTradeImagesToResponse(images []model.TradeImage) []TradeImageResponse {
+	result := make([]TradeImageResponse, 0, len(images))
+	for i := range images {
+		result = append(result, ConvertTradeImageToResponse(&images[i]))
+	}
+	return result
+}
+
+func ConvertTradeItemToResponse(item *model.TradeItem, isFavorited bool) *TradeItemResponse {
+	if item == nil {
+		return nil
+	}
+	resp := &TradeItemResponse{
+		ID:             item.ID,
+		UserID:         item.UserID,
+		TradeType:      string(item.TradeType),
+		Category:       string(item.Category),
+		Title:          item.Title,
+		Content:        item.Content,
+		Price:          item.Price,
+		OriginalPrice:  item.OriginalPrice,
+		Condition:      item.Condition,
+		Location:       item.Location,
+		Status:         string(item.Status),
+		ViewsCount:     item.ViewsCount,
+		FavoritesCount: item.FavoritesCount,
+		IsFavorited:    isFavorited,
+		Images:         ConvertTradeImagesToResponse(item.Images),
+		CreatedAt:      item.CreatedAt.Format("2006-01-02T15:04:05Z07:00"),
+		UpdatedAt:      item.UpdatedAt.Format("2006-01-02T15:04:05Z07:00"),
+	}
+	if len(item.Segments) > 0 {
+		resp.Segments = item.Segments
+	}
+	if item.User != nil {
+		resp.Author = ConvertUserToResponse(item.User)
+	}
+	return resp
+}
+
+func ConvertTradeItemsToResponse(items []*model.TradeItem, favoriteMap map[string]bool) []*TradeItemResponse {
+	result := make([]*TradeItemResponse, 0, len(items))
+	for _, item := range items {
+		isFav := favoriteMap[item.ID]
+		result = append(result, ConvertTradeItemToResponse(item, isFav))
+	}
+	return result
+}
+
+func ConvertTradeItemToDetailResponse(item *model.TradeItem, isFavorited bool) *TradeItemDetailResponse {
+	if item == nil {
+		return nil
+	}
+	resp := &TradeItemDetailResponse{
+		ID:             item.ID,
+		UserID:         item.UserID,
+		TradeType:      string(item.TradeType),
+		Category:       string(item.Category),
+		Title:          item.Title,
+		Content:        item.Content,
+		Price:          item.Price,
+		OriginalPrice:  item.OriginalPrice,
+		Condition:      item.Condition,
+		Location:       item.Location,
+		Status:         string(item.Status),
+		ViewsCount:     item.ViewsCount,
+		FavoritesCount: item.FavoritesCount,
+		IsFavorited:    isFavorited,
+		Images:         ConvertTradeImagesToResponse(item.Images),
+		CreatedAt:      item.CreatedAt.Format("2006-01-02T15:04:05Z07:00"),
+		UpdatedAt:      item.UpdatedAt.Format("2006-01-02T15:04:05Z07:00"),
+	}
+	if len(item.Segments) > 0 {
+		resp.Segments = item.Segments
+	}
+	if item.User != nil {
+		resp.Author = ConvertUserToResponse(item.User)
+	}
+	return resp
+}