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

internal/model/trade.go

@@ -0,0 +1,112 @@
+package model
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+type TradeType string
+
+const (
+	TradeTypeSell TradeType = "sell"
+	TradeTypeBuy  TradeType = "buy"
+)
+
+type TradeCategory string
+
+const (
+	TradeCategoryDigital   TradeCategory = "digital"
+	TradeCategoryBook      TradeCategory = "book"
+	TradeCategoryClothing  TradeCategory = "clothing"
+	TradeCategoryDaily     TradeCategory = "daily"
+	TradeCategoryCosmetics TradeCategory = "cosmetics"
+	TradeCategorySport     TradeCategory = "sport"
+	TradeCategoryTicket    TradeCategory = "ticket"
+	TradeCategoryOther     TradeCategory = "other"
+)
+
+type TradeItemStatus string
+
+const (
+	TradeItemActive  TradeItemStatus = "active"
+	TradeItemSold    TradeItemStatus = "sold"
+	TradeItemBought  TradeItemStatus = "bought"
+	TradeItemOffline TradeItemStatus = "offline"
+)
+
+type TradeItem struct {
+	ID             string          `json:"id" gorm:"type:varchar(36);primaryKey"`
+	UserID         string          `json:"user_id" gorm:"type:varchar(36);index;not null"`
+	TradeType      TradeType       `json:"trade_type" gorm:"type:varchar(10);not null;index"`
+	Category       TradeCategory   `json:"category" gorm:"type:varchar(20);not null;index"`
+	Title          string          `json:"title" gorm:"type:varchar(100);not null"`
+	Content        string          `json:"content" gorm:"type:text"`
+	Segments       MessageSegments `json:"segments,omitempty" gorm:"type:text"`
+	User           *User           `json:"user,omitempty" gorm:"foreignKey:UserID"`
+	Images         []TradeImage    `json:"images" gorm:"foreignKey:TradeItemID"`
+	Price          *float64        `json:"price,omitempty" gorm:"type:decimal(10,2)"`
+	OriginalPrice  *float64        `json:"original_price,omitempty" gorm:"type:decimal(10,2)"`
+	Condition      string          `json:"condition,omitempty" gorm:"type:varchar(20)"`
+	Location       string          `json:"location,omitempty" gorm:"type:varchar(100)"`
+	Status         TradeItemStatus `json:"status" gorm:"type:varchar(10);not null;default:'active';index"`
+	ViewsCount     int             `json:"views_count" gorm:"default:0"`
+	FavoritesCount int             `json:"favorites_count" gorm:"default:0"`
+	DeletedAt      gorm.DeletedAt  `json:"-" gorm:"index"`
+	CreatedAt      time.Time       `json:"created_at" gorm:"autoCreateTime;index"`
+	UpdatedAt      time.Time       `json:"updated_at" gorm:"autoUpdateTime:false"`
+}
+
+func (t *TradeItem) BeforeCreate(tx *gorm.DB) error {
+	if t.ID == "" {
+		t.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (TradeItem) TableName() string {
+	return "trade_items"
+}
+
+type TradeImage struct {
+	ID              string    `json:"id" gorm:"type:varchar(36);primaryKey"`
+	TradeItemID     string    `json:"trade_item_id" gorm:"type:varchar(36);not null;index"`
+	URL             string    `json:"url" gorm:"type:text;not null"`
+	ThumbnailURL    string    `json:"thumbnail_url" gorm:"type:text"`
+	PreviewURL      string    `json:"preview_url" gorm:"type:text"`
+	PreviewURLLarge string    `json:"preview_url_large" gorm:"type:text"`
+	Width           int       `json:"width" gorm:"default:0"`
+	Height          int       `json:"height" gorm:"default:0"`
+	SortOrder       int       `json:"sort_order" gorm:"default:0"`
+	CreatedAt       time.Time `json:"created_at" gorm:"autoCreateTime"`
+}
+
+func (ti *TradeImage) BeforeCreate(tx *gorm.DB) error {
+	if ti.ID == "" {
+		ti.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (TradeImage) TableName() string {
+	return "trade_images"
+}
+
+type TradeFavorite struct {
+	ID          string    `json:"id" gorm:"type:varchar(36);primaryKey"`
+	TradeItemID string    `json:"trade_item_id" gorm:"type:varchar(36);not null;index;uniqueIndex:idx_trade_fav_item_user,priority:1"`
+	UserID      string    `json:"user_id" gorm:"type:varchar(36);not null;index;uniqueIndex:idx_trade_fav_item_user,priority:2"`
+	CreatedAt   time.Time `json:"created_at" gorm:"autoCreateTime"`
+}
+
+func (f *TradeFavorite) BeforeCreate(tx *gorm.DB) error {
+	if f.ID == "" {
+		f.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (TradeFavorite) TableName() string {
+	return "trade_favorites"
+}