feat(web+api): 贡献表单按 archive_kind 动态生成字段

- categories 接口返回 archive_kind
- 新增公开只读接口 /api/v1/kind-fields?kind=X 返回字段模板
- 投稿 payload 支持通用 attributes,审核通过写入 product.attributes (JSONB 合并)
- food_detail 仅在含食品数据时才 upsert (药品/3C/通用不再产生空行)
- 前端 Contribute 按所选品类 archive_kind 动态渲染字段
  (食品营养 / 药品 18 字段 / 3C / 通用),除商品名外均选填

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

api/internal/handler/handler.go

@@ -127,6 +127,7 @@ func (h *Handler) Router() http.Handler {
 			})
 			r.Get("/brands", h.ListBrands)
 			r.Get("/categories", h.ListCategories)
+			r.Get("/kind-fields", h.ListKindFields)
 			r.Get("/sources/{id}", h.SourceByID)
 			r.Get("/stats", h.Stats)
 		})
@@ -275,6 +276,21 @@ func (h *Handler) ListCategories(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]any{"items": items})
 }
 
+// ListKindFields returns the read-only spec field template for an archive kind,
+// used by the contribution form to render kind-specific inputs.
+func (h *Handler) ListKindFields(w http.ResponseWriter, r *http.Request) {
+	kind := strings.TrimSpace(r.URL.Query().Get("kind"))
+	if kind == "" {
+		writeError(w, r, http.StatusBadRequest, "bad_request", "missing kind")
+		return
+	}
+	items, err := h.store.ListKindFields(r.Context(), kind)
+	if h.handleErr(w, r, err) {
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"items": items, "kind": kind})
+}
+
 // SourceByID returns a single data source.
 func (h *Handler) SourceByID(w http.ResponseWriter, r *http.Request) {
 	src, err := h.store.SourceByID(r.Context(), chi.URLParam(r, "id"))