feat(M3): 只读 API 端点实现

- store: pgx 只读数据访问层(productByGTIN/ByID/search/nutriments/msrp/brands/categories/source)
- handler: 真实查询替换 501 占位, 统一分页 + 错误信封, MSRP 带免责声明无购买入口
- main: pgxpool 连接池接线
- search: 名称模糊 + 分类子树过滤(ltree <@)
- 测试: healthz/pageParams 单测 + DB-backed handler 集成测试(无库自动跳过)
- CI: Go job 增加 postgres service + migrate up, 实跑 DB 测试
- 依赖: pgx v5.7.2 (固定到兼容 go1.23 的版本)
- 本地实跑: 8 个端点对真实 OFF 数据返回正确(barcode/search/nutriments/msrp/brands/categories/source/404)

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

.github/workflows/ci.yml

@@ -12,12 +12,31 @@ jobs:
     defaults:
       run:
         working-directory: api
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_USER: opengoods
+          POSTGRES_PASSWORD: opengoods
+          POSTGRES_DB: opengoods
+        ports:
+          - "5432:5432"
+        options: >-
+          --health-cmd "pg_isready -U opengoods"
+          --health-interval 5s --health-timeout 5s --health-retries 10
+    env:
+      OPENGOODS_DATABASE_URL: postgres://opengoods:opengoods@localhost:5432/opengoods?sslmode=disable
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5
         with:
           go-version: "1.23"
           cache-dependency-path: api/go.sum
+      - name: Apply migrations
+        working-directory: .
+        run: |
+          go install -tags 'postgres' github.com/golang-migrate/migrate/v4/cmd/migrate@v4.18.1
+          migrate -path migrations -database "$OPENGOODS_DATABASE_URL" up
       - name: Verify gofmt
         run: test -z "$(gofmt -l .)"
       - run: go vet ./...