feat(barcode): admin barcode CRUD endpoints + UI; show extra barcodes publicly

Wire the multi-barcode store layer to HTTP and the operator console:

- adminhandler: add POST /products/{id}/barcodes, DELETE
  /products/{id}/barcodes/{barcodeID}, and POST .../primary. A barcode
  owned by another product returns 409 with the conflicting product
  (gtin/product_id/product_name); an invalid GTIN returns 400.
- admin-frontend: BarcodesCard on the product detail page lists all
  barcodes (primary starred), adds with type/pack-level/region, sets
  primary, and deletes; audit labels for the new actions.
- public-frontend: product detail surfaces non-primary barcodes so a
  case/region code resolves and is visible to consumers.

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

admin-frontend/src/types.ts

@@ -10,6 +10,15 @@ export interface ProductRow {
   updated_at: string;
 }
 
+export interface Barcode {
+  id: string;
+  gtin: string;
+  gtin_type: string;
+  pack_level: string;
+  region: string | null;
+  is_primary: boolean;
+}
+
 export interface ProductImage {
   id: string;
   url: string;
@@ -47,6 +56,7 @@ export interface ProductDetail {
   nutrition_basis: string | null;
   serving_size: string | null;
   nutri_score: string | null;
+  barcodes: Barcode[];
   images: ProductImage[];
   msrp: MSRP[];
   missing: string[];