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/testhelper.go

@@ -0,0 +1,160 @@
+//go:build mysql
+
+package mysql
+
+import (
+	"database/sql"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/pressly/goose/v3"
+	"github.com/stretchr/testify/require"
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+)
+
+type MySQLTestConfig struct {
+	Host     string
+	Port     int
+	User     string
+	Password string
+	Database string
+}
+
+func getMySQLTestConfig() *MySQLTestConfig {
+	return &MySQLTestConfig{
+		Host:     getEnvOrDefault("NEX_TEST_MYSQL_HOST", "localhost"),
+		Port:     getEnvOrDefaultInt("NEX_TEST_MYSQL_PORT", 13306),
+		User:     getEnvOrDefault("NEX_TEST_MYSQL_USER", "nex_test"),
+		Password: getEnvOrDefault("NEX_TEST_MYSQL_PASSWORD", "testpass"),
+		Database: getEnvOrDefault("NEX_TEST_MYSQL_DATABASE", "nex_test"),
+	}
+}
+
+func getEnvOrDefault(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}
+
+func getEnvOrDefaultInt(key string, defaultValue int) int {
+	if value := os.Getenv(key); value != "" {
+		var intValue int
+		if _, err := fmt.Sscanf(value, "%d", &intValue); err == nil {
+			return intValue
+		}
+	}
+	return defaultValue
+}
+
+func SkipIfMySQLUnavailable(t *testing.T) {
+	t.Helper()
+
+	cfg := getMySQLTestConfig()
+	dsn := fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=true&loc=Local",
+		cfg.User, cfg.Password, cfg.Host, cfg.Port, cfg.Database)
+
+	db, err := sql.Open("mysql", dsn)
+	if err != nil {
+		t.Skipf("MySQL 不可用: %v", err)
+	}
+	defer db.Close()
+
+	if err := db.Ping(); err != nil {
+		t.Skipf("MySQL 不可用: %v", err)
+	}
+}
+
+func SetupMySQLTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+
+	SkipIfMySQLUnavailable(t)
+
+	cfg := getMySQLTestConfig()
+	dsn := fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=true&loc=Local",
+		cfg.User, cfg.Password, cfg.Host, cfg.Port, cfg.Database)
+
+	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	require.NoError(t, err, "连接 MySQL 失败")
+
+	if err := runMigrations(db); err != nil {
+		require.NoError(t, err, "运行迁移失败")
+	}
+
+	if err := cleanupTables(db); err != nil {
+		require.NoError(t, err, "清理表数据失败")
+	}
+
+	sqlDB, err := db.DB()
+	require.NoError(t, err)
+	sqlDB.SetMaxIdleConns(10)
+	sqlDB.SetMaxOpenConns(100)
+	sqlDB.SetConnMaxLifetime(time.Hour)
+
+	t.Cleanup(func() {
+		time.Sleep(50 * time.Millisecond)
+		sqlDB, err := db.DB()
+		if err == nil {
+			sqlDB.Close()
+		}
+	})
+
+	return db
+}
+
+func cleanupTables(db *gorm.DB) error {
+	if err := db.Exec("SET FOREIGN_KEY_CHECKS = 0").Error; err != nil {
+		return err
+	}
+	if err := db.Exec("TRUNCATE TABLE usage_stats").Error; err != nil {
+		return err
+	}
+	if err := db.Exec("TRUNCATE TABLE models").Error; err != nil {
+		return err
+	}
+	if err := db.Exec("TRUNCATE TABLE providers").Error; err != nil {
+		return err
+	}
+	if err := db.Exec("SET FOREIGN_KEY_CHECKS = 1").Error; err != nil {
+		return err
+	}
+	return nil
+}
+
+func runMigrations(db *gorm.DB) error {
+	sqlDB, err := db.DB()
+	if err != nil {
+		return err
+	}
+
+	migrationsDir := getMigrationsDir()
+	if _, err := os.Stat(migrationsDir); os.IsNotExist(err) {
+		return fmt.Errorf("迁移目录不存在: %s", migrationsDir)
+	}
+
+	goose.SetDialect("mysql")
+	if err := goose.Up(sqlDB, migrationsDir); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func getMigrationsDir() string {
+	_, filename, _, ok := runtime.Caller(0)
+	if ok {
+		dir := filepath.Join(filepath.Dir(filename), "..", "..", "migrations", "mysql")
+		if abs, err := filepath.Abs(dir); err == nil {
+			return abs
+		}
+	}
+	return "./migrations/mysql"
+}