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>

api/cmd/server/main.go

@@ -2,20 +2,39 @@
 package main
 
 import (
+	"context"
 	"log"
 	"net/http"
 	"time"
 
+	"github.com/jackc/pgx/v5/pgxpool"
+
 	"github.com/baicai2026-baicai/goods/api/internal/config"
 	"github.com/baicai2026-baicai/goods/api/internal/handler"
+	"github.com/baicai2026-baicai/goods/api/internal/store"
 )
 
 func main() {
 	cfg := config.Load()
 
+	ctx := context.Background()
+	pool, err := pgxpool.New(ctx, cfg.DatabaseURL)
+	if err != nil {
+		log.Fatalf("failed to create db pool: %v", err)
+	}
+	defer pool.Close()
+
+	pingCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+	defer cancel()
+	if err := pool.Ping(pingCtx); err != nil {
+		log.Printf("warning: database not reachable at startup: %v", err)
+	}
+
+	h := handler.New(store.New(pool))
+
 	srv := &http.Server{
 		Addr:              cfg.Addr,
-		Handler:           handler.Router(),
+		Handler:           h.Router(),
 		ReadHeaderTimeout: 10 * time.Second,
 	}