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/concurrent_test.go

@@ -0,0 +1,158 @@
+//go:build mysql
+
+package mysql
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"nex/backend/internal/config"
+	"nex/backend/internal/repository"
+)
+
+func TestConcurrent_UsageStatsRecord(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	statsRepo := repository.NewStatsRepository(db)
+
+	providerID := "concurrent-test-provider"
+	modelName := "gpt-4"
+
+	concurrency := 10
+	var wg sync.WaitGroup
+	wg.Add(concurrency)
+
+	errChan := make(chan error, concurrency)
+
+	for i := 0; i < concurrency; i++ {
+		go func() {
+			defer wg.Done()
+			err := statsRepo.Record(providerID, modelName)
+			if err != nil {
+				errChan <- err
+			}
+		}()
+	}
+
+	wg.Wait()
+	close(errChan)
+
+	var errorCount int
+	uniqueErrors := make(map[string]int)
+	for err := range errChan {
+		errorCount++
+		uniqueErrors[err.Error()]++
+	}
+
+	t.Logf("并发 %d 次，错误 %d 次", concurrency, errorCount)
+	for errMsg, count := range uniqueErrors {
+		t.Logf("  错误: %s (出现 %d 次)", errMsg, count)
+	}
+
+	var stats config.UsageStats
+	err := db.Where("provider_id = ? AND model_name = ?", providerID, modelName).
+		First(&stats).Error
+	require.NoError(t, err, "应能查到 usage_stats 记录")
+
+	successCount := concurrency - errorCount
+	t.Logf("成功次数: %d, 最终 request_count: %d", successCount, stats.RequestCount)
+
+	assert.Equal(t, concurrency, stats.RequestCount, "request_count 应等于并发数，无数据丢失或重复")
+}
+
+func TestConcurrent_ProviderCreate(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	providerID := "concurrent-provider-id"
+	concurrency := 10
+
+	var wg sync.WaitGroup
+	wg.Add(concurrency)
+
+	successCount := 0
+	var mu sync.Mutex
+
+	for i := 0; i < concurrency; i++ {
+		go func() {
+			defer wg.Done()
+
+			provider := config.Provider{
+				ID:      providerID,
+				Name:    "Concurrent Provider",
+				APIKey:  "test-key",
+				BaseURL: "https://test.com",
+				Enabled: true,
+			}
+
+			err := db.Create(&provider).Error
+			if err == nil {
+				mu.Lock()
+				successCount++
+				mu.Unlock()
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	assert.Equal(t, 1, successCount, "仅 1 个创建应成功")
+
+	var count int64
+	db.Model(&config.Provider{}).Where("id = ?", providerID).Count(&count)
+	assert.Equal(t, int64(1), count, "最终应有 1 条记录")
+}
+
+func TestConcurrent_ModelCreate(t *testing.T) {
+	db := SetupMySQLTestDB(t)
+
+	provider := config.Provider{
+		ID:      "concurrent-model-provider",
+		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")
+
+	modelName := "gpt-4-concurrent"
+	concurrency := 10
+
+	var wg sync.WaitGroup
+	wg.Add(concurrency)
+
+	successCount := 0
+	var mu sync.Mutex
+
+	for i := 0; i < concurrency; i++ {
+		go func(idx int) {
+			defer wg.Done()
+
+			model := config.Model{
+				ID:         uuid.New().String(),
+				ProviderID: provider.ID,
+				ModelName:  modelName,
+				Enabled:    true,
+			}
+
+			err := db.Create(&model).Error
+			if err == nil {
+				mu.Lock()
+				successCount++
+				mu.Unlock()
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	assert.Equal(t, 1, successCount, "仅 1 个创建应成功")
+
+	var count int64
+	db.Model(&config.Model{}).Where("provider_id = ? AND model_name = ?", provider.ID, modelName).Count(&count)
+	assert.Equal(t, int64(1), count, "最终应有 1 条记录")
+}