fix: replace MySQL NOW() with SQLite-compatible datetime('now')

- Set function: use GORM clause.OnConflict for cross-database upsert
- BatchSet function: replace NOW() with datetime('now')
- Add tests for Set and BatchSet (both now 100%/85.7% covered)

internal/repository/custom_field_repository_test.go

@@ -330,3 +330,87 @@ func TestUserCustomFieldValueRepository_DeleteByUserID(t *testing.T) {
 		t.Errorf("用户2的字段值应该保留, got %d", len(values2))
 	}
 }
+
+// TestUserCustomFieldValueRepository_Set 测试设置用户字段值（upsert）
+func TestUserCustomFieldValueRepository_Set(t *testing.T) {
+	db := setupCustomFieldTestDB(t)
+	fieldRepo := NewCustomFieldRepository(db)
+	valueRepo := NewUserCustomFieldValueRepository(db)
+	ctx := context.Background()
+
+	// 先创建字段
+	field := &domain.CustomField{
+		Name:     "user-field",
+		FieldKey: "user_field_key",
+		Type:     domain.CustomFieldTypeString,
+	}
+	fieldRepo.Create(ctx, field)
+
+	// 设置用户字段值
+	err := valueRepo.Set(ctx, 1, field.ID, field.FieldKey, "test_value")
+	if err != nil {
+		t.Fatalf("Set() error = %v", err)
+	}
+
+	// 验证值已设置
+	found, err := valueRepo.GetByUserIDAndFieldKey(ctx, 1, "user_field_key")
+	if err != nil {
+		t.Fatalf("GetByUserIDAndFieldKey() error = %v", err)
+	}
+	if found.Value != "test_value" {
+		t.Errorf("Value = %v, want test_value", found.Value)
+	}
+
+	// 测试 upsert（更新已存在的值）
+	err = valueRepo.Set(ctx, 1, field.ID, field.FieldKey, "updated_value")
+	if err != nil {
+		t.Fatalf("Set() upsert error = %v", err)
+	}
+
+	found2, _ := valueRepo.GetByUserIDAndFieldKey(ctx, 1, "user_field_key")
+	if found2.Value != "updated_value" {
+		t.Errorf("Value after upsert = %v, want updated_value", found2.Value)
+	}
+}
+
+// TestUserCustomFieldValueRepository_BatchSet 测试批量设置用户字段值
+func TestUserCustomFieldValueRepository_BatchSet(t *testing.T) {
+	db := setupCustomFieldTestDB(t)
+	fieldRepo := NewCustomFieldRepository(db)
+	valueRepo := NewUserCustomFieldValueRepository(db)
+	ctx := context.Background()
+
+	// 先创建字段
+	field1 := &domain.CustomField{Name: "batch1", FieldKey: "batch_key1", Type: domain.CustomFieldTypeString}
+	field2 := &domain.CustomField{Name: "batch2", FieldKey: "batch_key2", Type: domain.CustomFieldTypeNumber}
+	fieldRepo.Create(ctx, field1)
+	fieldRepo.Create(ctx, field2)
+
+	values := map[string]string{
+		"batch_key1": "batch_value1",
+		"batch_key2": "batch_value2",
+	}
+
+	err := valueRepo.BatchSet(ctx, 1, values)
+	if err != nil {
+		t.Fatalf("BatchSet() error = %v", err)
+	}
+
+	v1, _ := valueRepo.GetByUserIDAndFieldKey(ctx, 1, "batch_key1")
+	v2, _ := valueRepo.GetByUserIDAndFieldKey(ctx, 1, "batch_key2")
+	if v1.Value != "batch_value1" || v2.Value != "batch_value2" {
+		t.Error("BatchSet values not set correctly")
+	}
+}
+
+// TestUserCustomFieldValueRepository_BatchSet_Empty 测试空批量设置
+func TestUserCustomFieldValueRepository_BatchSet_Empty(t *testing.T) {
+	db := setupCustomFieldTestDB(t)
+	valueRepo := NewUserCustomFieldValueRepository(db)
+	ctx := context.Background()
+
+	err := valueRepo.BatchSet(ctx, 1, map[string]string{})
+	if err != nil {
+		t.Fatalf("BatchSet() error = %v", err)
+	}
+}