fix(backend): dbErr 503 degradation for connection-class DB errors

backend/internal/api/auth.go

@@ -2,7 +2,6 @@ package api
 
 import (
 	"errors"
-	"log"
 	"net/http"
 	"time"
 
@@ -28,8 +27,7 @@ func (a *api) login(c *gin.Context) {
 	u, qerr := a.st.GetUserByName(c, req.Username)
 	if qerr != nil {
 		if !errors.Is(qerr, pgx.ErrNoRows) {
-			log.Printf("db: %v", qerr)
-			err(c, http.StatusInternalServerError, "internal", "db error")
+			dbErr(c, qerr)
 			return
 		}
 		// 用户不存在也走一次 bcrypt，防用户名枚举时序差