feat: 新增 MySQL 专项测试能力

- 新增 backend/tests/mysql/ 目录，包含 Docker Compose 配置和测试文件
- 新增 Makefile 命令: test-mysql, test-mysql-up, test-mysql-down, test-mysql-quick
- 使用 build tag 控制测试启用，默认不运行
- 测试覆盖: 迁移正确性、外键约束、UNIQUE 约束、并发写入
- 发现 statsRepo.Record 存在并发 bug（检查-然后-操作竞态条件）

backend/tests/mysql/constraint_test.go

@@ -0,0 +1,130 @@
+//go:build mysql
+
+package mysql
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"gorm.io/gorm"
+
+	"nex/backend/internal/config"
+)
+
+func TestConstraint_ForeignKeyEnforced(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	model := config.Model{
+		ID:         "test-model-id",
+		ProviderID: "non-existent-provider",
+		ModelName:  "gpt-4",
+		Enabled:    true,
+	}
+
+	err := db.Create(&model).Error
+	assert.Error(t, err, "创建 model 时 provider_id 不存在应失败")
+	assert.Contains(t, err.Error(), "foreign key constraint", "错误应为外键约束错误")
+}
+
+func TestConstraint_CascadeDelete(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	provider := config.Provider{
+		ID:      "test-provider-cascade",
+		Name:    "Test Provider",
+		APIKey:  "test-key",
+		BaseURL: "https://test.com",
+		Enabled: true,
+	}
+	err := db.Create(&provider).Error
+	require.NoError(t, err, "创建 provider 应成功")
+
+	model := config.Model{
+		ID:         "test-model-cascade",
+		ProviderID: provider.ID,
+		ModelName:  "gpt-4",
+		Enabled:    true,
+	}
+	err = db.Create(&model).Error
+	require.NoError(t, err, "创建 model 应成功")
+
+	err = db.Delete(&provider).Error
+	require.NoError(t, err, "删除 provider 应成功")
+
+	var count int64
+	err = db.Model(&config.Model{}).Where("provider_id = ?", provider.ID).Count(&count).Error
+	require.NoError(t, err)
+	assert.Equal(t, int64(0), count, "删除 provider 后其 models 应被级联删除")
+}
+
+func TestConstraint_UniqueProviderModel(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	provider := config.Provider{
+		ID:      "test-provider-unique",
+		Name:    "Test Provider",
+		APIKey:  "test-key",
+		BaseURL: "https://test.com",
+		Enabled: true,
+	}
+	err := db.Create(&provider).Error
+	require.NoError(t, err, "创建 provider 应成功")
+
+	model1 := config.Model{
+		ID:         "test-model-unique-1",
+		ProviderID: provider.ID,
+		ModelName:  "gpt-4",
+		Enabled:    true,
+	}
+	err = db.Create(&model1).Error
+	require.NoError(t, err, "创建第一个 model 应成功")
+
+	model2 := config.Model{
+		ID:         "test-model-unique-2",
+		ProviderID: provider.ID,
+		ModelName:  "gpt-4",
+		Enabled:    true,
+	}
+	err = db.Create(&model2).Error
+	assert.Error(t, err, "创建相同 (provider_id, model_name) 的 model 应失败")
+	assert.True(t, errors.Is(err, gorm.ErrDuplicatedKey) || 
+		(err != nil && (err.Error() == "Error 1062" || containsDuplicateError(err.Error()))),
+		"错误应为唯一约束错误")
+}
+
+func TestConstraint_UniqueUsageStats(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	today := time.Now().Format("2006-01-02")
+	todayTime, _ := time.Parse("2006-01-02", today)
+
+	providerID := "test-provider-unique-stats"
+
+	stats1 := config.UsageStats{
+		ProviderID:   providerID,
+		ModelName:    "gpt-4",
+		RequestCount: 10,
+		Date:         todayTime,
+	}
+	err := db.Create(&stats1).Error
+	require.NoError(t, err, "创建第一个 usage_stats 应成功")
+
+	stats2 := config.UsageStats{
+		ProviderID:   providerID,
+		ModelName:    "gpt-4",
+		RequestCount: 20,
+		Date:         todayTime,
+	}
+	err = db.Create(&stats2).Error
+	assert.Error(t, err, "创建相同 (provider_id, model_name, date) 的 usage_stats 应失败")
+	assert.True(t, errors.Is(err, gorm.ErrDuplicatedKey) || 
+		(err != nil && (err.Error() == "Error 1062" || containsDuplicateError(err.Error()))),
+		"错误应为唯一约束错误")
+}
+
+func containsDuplicateError(errStr string) bool {
+	return len(errStr) > 0 && (errStr[0:8] == "Error 10" || errStr[0:5] == "Dupli")
+}