feat(admin): 品牌管理 + 新建商品

品牌管理：列出品牌及引用商品数，支持改名、合并重复品牌（把源品牌的商品并入目标后删除源）、删除未被引用的品牌。新建商品：商品列表新增「新建商品」入口，填写名称/条码/品牌/品类后创建并进入详情页继续补全。

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/internal/adminhandler/handler.go

@@ -65,6 +65,7 @@ func (h *Handler) Router() http.Handler {
 			r.Use(h.authn.Middleware)
 			r.Get("/api/me", h.Me)
 			r.Get("/api/products", h.ListProducts)
+			r.Post("/api/products", h.CreateProduct)
 			r.Get("/api/products/{id}", h.GetProduct)
 			r.Put("/api/products/{id}", h.UpdateProduct)
 			r.Get("/api/products/{id}/audit", h.ListAudit)
@@ -76,6 +77,10 @@ func (h *Handler) Router() http.Handler {
 			r.Delete("/api/products/{id}/barcodes/{barcodeID}", h.DeleteBarcode)
 			r.Post("/api/products/{id}/barcodes/{barcodeID}/primary", h.SetPrimaryBarcode)
 			r.Get("/api/brands", h.ListBrands)
+			r.Post("/api/brands", h.CreateBrand)
+			r.Put("/api/brands/{id}", h.UpdateBrand)
+			r.Post("/api/brands/{id}/merge", h.MergeBrands)
+			r.Delete("/api/brands/{id}", h.DeleteBrand)
 			r.Get("/api/categories", h.ListCategories)
 			r.Post("/api/categories", h.CreateCategory)
 			r.Put("/api/categories/{id}", h.UpdateCategory)
@@ -175,6 +180,29 @@ func (h *Handler) GetProduct(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, d)
 }
 
+// CreateProduct adds a new product with core fields; the rest is filled in via
+// the detail editor.
+func (h *Handler) CreateProduct(w http.ResponseWriter, r *http.Request) {
+	var in adminstore.ProductInput
+	if err := json.NewDecoder(r.Body).Decode(&in); err != nil {
+		writeError(w, http.StatusBadRequest, "bad_request", "invalid body")
+		return
+	}
+	if strings.TrimSpace(in.Name) == "" {
+		writeError(w, http.StatusBadRequest, "bad_request", "名称不能为空")
+		return
+	}
+	d, err := h.store.CreateProduct(r.Context(), auth.UserFrom(r.Context()), in)
+	if errors.Is(err, adminstore.ErrDuplicateGTIN) {
+		writeError(w, http.StatusConflict, "duplicate_gtin", "该条码(GTIN)已被其它商品使用")
+		return
+	}
+	if h.handleErr(w, err) {
+		return
+	}
+	writeJSON(w, http.StatusCreated, d)
+}
+
 // UpdateProduct applies an edit.
 func (h *Handler) UpdateProduct(w http.ResponseWriter, r *http.Request) {
 	var in adminstore.ProductInput
@@ -399,6 +427,96 @@ func (h *Handler) handleCategoryErr(w http.ResponseWriter, err error) bool {
 	return h.handleErr(w, err)
 }
 
+type brandInput struct {
+	Name string `json:"name"`
+}
+
+type brandMergeInput struct {
+	TargetID string `json:"target_id"`
+}
+
+// CreateBrand adds a brand.
+func (h *Handler) CreateBrand(w http.ResponseWriter, r *http.Request) {
+	var in brandInput
+	if err := json.NewDecoder(r.Body).Decode(&in); err != nil {
+		writeError(w, http.StatusBadRequest, "bad_request", "invalid body")
+		return
+	}
+	if strings.TrimSpace(in.Name) == "" {
+		writeError(w, http.StatusBadRequest, "bad_request", "品牌名称不能为空")
+		return
+	}
+	b, err := h.store.CreateBrand(r.Context(), auth.UserFrom(r.Context()), in.Name)
+	if h.handleBrandErr(w, err) {
+		return
+	}
+	writeJSON(w, http.StatusCreated, b)
+}
+
+// UpdateBrand renames a brand.
+func (h *Handler) UpdateBrand(w http.ResponseWriter, r *http.Request) {
+	var in brandInput
+	if err := json.NewDecoder(r.Body).Decode(&in); err != nil {
+		writeError(w, http.StatusBadRequest, "bad_request", "invalid body")
+		return
+	}
+	if strings.TrimSpace(in.Name) == "" {
+		writeError(w, http.StatusBadRequest, "bad_request", "品牌名称不能为空")
+		return
+	}
+	b, err := h.store.UpdateBrand(r.Context(), chi.URLParam(r, "id"), auth.UserFrom(r.Context()), in.Name)
+	if h.handleBrandErr(w, err) {
+		return
+	}
+	writeJSON(w, http.StatusOK, b)
+}
+
+// MergeBrands folds one brand's products into another, then deletes the source.
+func (h *Handler) MergeBrands(w http.ResponseWriter, r *http.Request) {
+	var in brandMergeInput
+	if err := json.NewDecoder(r.Body).Decode(&in); err != nil {
+		writeError(w, http.StatusBadRequest, "bad_request", "invalid body")
+		return
+	}
+	if strings.TrimSpace(in.TargetID) == "" {
+		writeError(w, http.StatusBadRequest, "bad_request", "请选择合并目标品牌")
+		return
+	}
+	b, err := h.store.MergeBrands(r.Context(), chi.URLParam(r, "id"), in.TargetID, auth.UserFrom(r.Context()))
+	if h.handleBrandErr(w, err) {
+		return
+	}
+	writeJSON(w, http.StatusOK, b)
+}
+
+// DeleteBrand removes a brand no product references.
+func (h *Handler) DeleteBrand(w http.ResponseWriter, r *http.Request) {
+	err := h.store.DeleteBrand(r.Context(), chi.URLParam(r, "id"), auth.UserFrom(r.Context()))
+	if h.handleBrandErr(w, err) {
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]string{"status": "deleted"})
+}
+
+// handleBrandErr maps brand-specific errors to client statuses.
+func (h *Handler) handleBrandErr(w http.ResponseWriter, err error) bool {
+	if err == nil {
+		return false
+	}
+	switch {
+	case errors.Is(err, adminstore.ErrDuplicateBrand):
+		writeError(w, http.StatusConflict, "duplicate_brand", "该品牌名称已存在")
+		return true
+	case errors.Is(err, adminstore.ErrBrandInUse):
+		writeError(w, http.StatusConflict, "in_use", "仍有商品使用该品牌，请先改用其它品牌或合并")
+		return true
+	case errors.Is(err, adminstore.ErrInvalidMerge):
+		writeError(w, http.StatusBadRequest, "invalid_merge", "合并目标无效（不存在或与源品牌相同）")
+		return true
+	}
+	return h.handleErr(w, err)
+}
+
 // ---------- submissions ----------
 
 // CreateSubmission accepts an anonymous public contribution into the queue.