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/handler/download_handler.go

@@ -0,0 +1,87 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"wx_service/internal/marketing/service"
+	"wx_service/internal/middleware"
+	"wx_service/internal/model"
+)
+
+type DownloadHandler struct {
+	svc *service.DownloadService
+}
+
+func NewDownloadHandler(svc *service.DownloadService) *DownloadHandler {
+	return &DownloadHandler{svc: svc}
+}
+
+func (h *DownloadHandler) Create(c *gin.Context) {
+	user := middleware.MustCurrentUser(c)
+
+	var req service.CreateDownloadRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, model.Error(http.StatusBadRequest, "请求参数错误"))
+		return
+	}
+
+	dl, err := h.svc.Create(user.ID, req)
+	if err != nil {
+		if service.IsBadRequestError(err) {
+			c.JSON(http.StatusBadRequest, model.Error(http.StatusBadRequest, err.Error()))
+			return
+		}
+		c.JSON(http.StatusInternalServerError, model.Error(http.StatusInternalServerError, "服务器错误"))
+		return
+	}
+
+	c.JSON(http.StatusOK, model.Success(dl))
+}
+
+type adCallbackRequest struct {
+	DownloadID uint `json:"download_id" binding:"required"`
+}
+
+func (h *DownloadHandler) AdCallback(c *gin.Context) {
+	var req adCallbackRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, model.Error(http.StatusBadRequest, "请求参数错误"))
+		return
+	}
+
+	if err := h.svc.MarkAdCompleted(req.DownloadID); err != nil {
+		if service.IsNotFoundError(err) {
+			c.JSON(http.StatusNotFound, model.Error(http.StatusNotFound, "记录不存在"))
+			return
+		}
+		c.JSON(http.StatusInternalServerError, model.Error(http.StatusInternalServerError, "服务器错误"))
+		return
+	}
+
+	c.JSON(http.StatusOK, model.Success(nil))
+}
+
+func (h *DownloadHandler) ListByUser(c *gin.Context) {
+	user := middleware.MustCurrentUser(c)
+	page := parseIntQuery(c, "page", 1)
+	pageSize := parseIntQuery(c, "page_size", 20)
+
+	resp, err := h.svc.ListByUser(user.ID, page, pageSize)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, model.Error(http.StatusInternalServerError, "服务器错误"))
+		return
+	}
+
+	c.JSON(http.StatusOK, model.Success(resp))
+}
+
+func (h *DownloadHandler) AdminStats(c *gin.Context) {
+	stats, err := h.svc.GetStats()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, model.Error(http.StatusInternalServerError, "服务器错误"))
+		return
+	}
+	c.JSON(http.StatusOK, model.Success(stats))
+}