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

backend/internal/api/router_test.go

@@ -1,11 +1,18 @@
 package api
 
 import (
+	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"syscall"
 	"testing"
 	"time"
 
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/puddle/v2"
+
 	"booklib/internal/config"
 	"booklib/internal/redispkg"
 )
@@ -23,3 +30,23 @@ func TestHealthz(t *testing.T) {
 		t.Fatalf("healthz = %d, want 200", w.Code)
 	}
 }
+
+func TestDBErrStatus(t *testing.T) {
+	for _, tc := range []struct {
+		e    error
+		want int
+	}{
+		{&pgconn.PgError{Code: "57P01"}, http.StatusServiceUnavailable}, // PG 停机:池内连接被服务端断
+		{&pgconn.PgError{Code: "08006"}, http.StatusServiceUnavailable},
+		{fmt.Errorf("dial: %w", syscall.ECONNREFUSED), http.StatusServiceUnavailable},
+		{puddle.ErrClosedPool, http.StatusServiceUnavailable},
+		{errors.New("boom"), http.StatusInternalServerError},
+	} {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		dbErr(c, tc.e)
+		if w.Code != tc.want {
+			t.Errorf("dbErr(%v) = %d, want %d", tc.e, w.Code, tc.want)
+		}
+	}
+}