feat(barcode): 多条码管理（后端+迁移+GTIN校验）WIP

- 迁移 0007: 新增 product_barcode 表（一品多码），回填旧 product.gtin 为主码，
  全局唯一索引保证「一码一品」，每品至多一个主码
- internal/gtin: GS1 GTIN-8/12/13/14 校验（校验位 + 拒收店内码/变量重量码/优惠券码）
- 公开只读 API: 任一条码命中商品、详情返回 barcodes、搜索匹配条码
- adminstore: 商品详情含 barcodes；新增 AddBarcode/DeleteBarcode/SetPrimaryBarcode，
  一码命中其他商品返回 ConflictError 供后台去重

待办（按用户要求暂停）: 后台 handler 路由、投稿/审核多条码、前后端 UI

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

migrations/0007_product_barcode.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS product_barcode;

migrations/0007_product_barcode.up.sql

@@ -0,0 +1,36 @@
+-- Multi-barcode support: one product can carry many GS1 barcodes
+-- (consumer unit EAN-13/UPC, case ITF-14, regional re-labels, etc.).
+-- product.gtin is kept as the denormalized "primary" barcode for
+-- backward compatibility and is mirrored from the is_primary row here.
+
+CREATE TABLE product_barcode (
+    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    product_id UUID NOT NULL REFERENCES product(id) ON DELETE CASCADE,
+    gtin       VARCHAR(14) NOT NULL,
+    gtin_type  VARCHAR(8) NOT NULL DEFAULT 'EAN13',
+    pack_level VARCHAR(8) NOT NULL DEFAULT 'each',
+    region     VARCHAR(8),
+    is_primary BOOLEAN NOT NULL DEFAULT false,
+    source_id  UUID REFERENCES source(id),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT product_barcode_type_chk CHECK (gtin_type IN ('EAN8','UPC','EAN13','ITF14','GTIN14')),
+    CONSTRAINT product_barcode_pack_chk CHECK (pack_level IN ('each','case','pallet'))
+);
+
+-- A barcode is globally unique: one code maps to exactly one product.
+CREATE UNIQUE INDEX idx_product_barcode_gtin ON product_barcode (gtin);
+CREATE INDEX idx_product_barcode_product ON product_barcode (product_id);
+-- At most one primary barcode per product.
+CREATE UNIQUE INDEX idx_product_barcode_primary ON product_barcode (product_id) WHERE is_primary;
+
+-- Backfill: lift each product's existing gtin into the new table as primary.
+INSERT INTO product_barcode (product_id, gtin, gtin_type, pack_level, is_primary)
+SELECT id, gtin,
+       CASE WHEN length(gtin) = 8 THEN 'EAN8'
+            WHEN length(gtin) = 12 THEN 'UPC'
+            WHEN length(gtin) = 14 THEN 'GTIN14'
+            ELSE 'EAN13' END,
+       'each', true
+FROM product
+WHERE gtin IS NOT NULL AND gtin <> ''
+ON CONFLICT (gtin) DO NOTHING;