feat(marketing): 新增营销图管理模块

- 新增 marketing 模块：model/repository/service/handler 四层架构
- 数据模型：marketing_categories、marketing_templates、marketing_user_downloads
- 小程序端接口：分类列表、模板列表/详情、下载记录、广告回调
- 管理后台接口：分类/模板 CRUD、下载统计（X-Admin-Token 鉴权）
- 路由注册：接入现有 AuthMiddleware，新增 AdminTokenMiddleware
- Web 管理后台：单页面 Vue3 + Element Plus（分类管理、模板管理、数据概览）

Closes #37, #38, #39, #40

Made-with: Cursor

internal/marketing/repository/download_repo.go

@@ -0,0 +1,98 @@
+package repository
+
+import (
+	"errors"
+	"fmt"
+
+	"gorm.io/gorm"
+
+	"wx_service/internal/marketing/model"
+)
+
+var ErrDownloadNotFound = errors.New("marketing download not found")
+
+type DownloadRepository struct {
+	db *gorm.DB
+}
+
+func NewDownloadRepository(db *gorm.DB) *DownloadRepository {
+	return &DownloadRepository{db: db}
+}
+
+func (r *DownloadRepository) Create(dl *model.MarketingDownload) error {
+	if err := r.db.Create(dl).Error; err != nil {
+		return fmt.Errorf("create marketing download: %w", err)
+	}
+	return nil
+}
+
+func (r *DownloadRepository) FindByID(id uint) (*model.MarketingDownload, error) {
+	var dl model.MarketingDownload
+	err := r.db.First(&dl, id).Error
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, ErrDownloadNotFound
+		}
+		return nil, fmt.Errorf("find marketing download: %w", err)
+	}
+	return &dl, nil
+}
+
+func (r *DownloadRepository) MarkAdCompleted(id uint) error {
+	tx := r.db.Model(&model.MarketingDownload{}).Where("id = ?", id).Update("ad_completed", true)
+	if tx.Error != nil {
+		return fmt.Errorf("mark ad completed: %w", tx.Error)
+	}
+	if tx.RowsAffected == 0 {
+		return ErrDownloadNotFound
+	}
+	return nil
+}
+
+func (r *DownloadRepository) FindByUser(userID uint, page, pageSize int) ([]model.MarketingDownload, int64, error) {
+	if page <= 0 {
+		page = 1
+	}
+	if pageSize <= 0 {
+		pageSize = 20
+	}
+	if pageSize > 100 {
+		pageSize = 100
+	}
+
+	query := r.db.Model(&model.MarketingDownload{}).Where("user_id = ?", userID)
+
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, fmt.Errorf("count marketing downloads: %w", err)
+	}
+
+	var downloads []model.MarketingDownload
+	offset := (page - 1) * pageSize
+	if err := query.Preload("Template").Order("id DESC").Offset(offset).Limit(pageSize).Find(&downloads).Error; err != nil {
+		return nil, 0, fmt.Errorf("list marketing downloads: %w", err)
+	}
+
+	return downloads, total, nil
+}
+
+type DownloadStats struct {
+	TotalDownloads int64
+	TodayDownloads int64
+}
+
+func (r *DownloadRepository) GetStats() (*DownloadStats, error) {
+	var total int64
+	if err := r.db.Model(&model.MarketingDownload{}).Count(&total).Error; err != nil {
+		return nil, fmt.Errorf("count total downloads: %w", err)
+	}
+
+	var today int64
+	if err := r.db.Model(&model.MarketingDownload{}).
+		Where("DATE(created_at) = CURDATE()").
+		Count(&today).Error; err != nil {
+		return nil, fmt.Errorf("count today downloads: %w", err)
+	}
+
+	return &DownloadStats{TotalDownloads: total, TodayDownloads: today}, nil
+}