feat: main.go 条件初始化 PostgreSQL 连接池

- 添加 store 包导入
- 当 storage.driver 为 postgres 时创建 pgxpool
- 使用 defer 确保连接池正确关闭
- 添加日志记录连接状态

backend/cmd/server/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/hhs/camtalk/internal/logger"
 	"github.com/hhs/camtalk/internal/orchestrator"
 	"github.com/hhs/camtalk/internal/session"
+	"github.com/hhs/camtalk/internal/store"
 	"github.com/hhs/camtalk/internal/ws"
 )
 
@@ -44,6 +45,21 @@ func main() {
 		"addr", cfg.Server.Addr(),
 	)
 
+	// 初始化存储层（条件初始化 PostgreSQL）
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	if cfg.Storage.Driver == "postgres" {
+		pool, err := store.NewPostgresPool(ctx, cfg.Storage.DSN)
+		if err != nil {
+			logger.Log.Fatalw("failed to connect to postgres", "error", err)
+		}
+		defer pool.Close()
+		logger.Log.Infow("postgres connected", "driver", cfg.Storage.Driver)
+		// TODO: Phase 2 - 初始化 UserRepository 和 MessageRepository
+		_ = pool
+	}
+
 	// 初始化 Session Manager（MVP 默认内存实现）
 	var sessionMgr session.Manager
 	// TODO: 当 Redis 配置非空时切换为 RedisManager