fix: 修复 statsRepo 并发竞态条件，使用 upsert 保证原子性

- 使用 GORM clause.OnConflict 替代事务包装
- Record 和 BatchUpdate 方法改用 upsert 模式
- 修复 UsageStats 的 GORM struct tag，确保 AutoMigrate 创建正确的 UNIQUE 约束
- 更新 usage-statistics spec 以反映 upsert 操作

MySQL 并发测试验证：10 并发调用 → request_count = 10

backend/internal/config/models.go

@@ -29,8 +29,8 @@ type Model struct {
 // UsageStats 用量统计
 type UsageStats struct {
 	ID           uint      `gorm:"primaryKey;autoIncrement" json:"id"`
-	ProviderID   string    `gorm:"not null;index" json:"provider_id"`
-	ModelName    string    `gorm:"not null;index" json:"model_name"`
+	ProviderID   string    `gorm:"not null;index;uniqueIndex:idx_provider_model_date" json:"provider_id"`
+	ModelName    string    `gorm:"not null;index;uniqueIndex:idx_provider_model_date" json:"model_name"`
 	RequestCount int       `gorm:"default:0" json:"request_count"`
 	Date         time.Time `gorm:"type:date;not null;uniqueIndex:idx_provider_model_date" json:"date"`
 }

backend/internal/repository/stats_repo_impl.go

@@ -1,10 +1,10 @@
 package repository
 
 import (
-	"errors"
 	"time"
 
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 
 	"nex/backend/internal/config"
 	"nex/backend/internal/domain"
@@ -22,47 +22,43 @@ func (r *statsRepository) Record(providerID, modelName string) error {
 	today := time.Now().Format("2006-01-02")
 	todayTime, _ := time.Parse("2006-01-02", today)
 
-	return r.db.Transaction(func(tx *gorm.DB) error {
-		var stats config.UsageStats
-		err := tx.Where("provider_id = ? AND model_name = ? AND date = ?",
-			providerID, modelName, todayTime).First(&stats).Error
+	stats := config.UsageStats{
+		ProviderID:   providerID,
+		ModelName:    modelName,
+		RequestCount: 1,
+		Date:         todayTime,
+	}
 
-		if errors.Is(err, gorm.ErrRecordNotFound) {
-			stats = config.UsageStats{
-				ProviderID:   providerID,
-				ModelName:    modelName,
-				RequestCount: 1,
-				Date:         todayTime,
-			}
-			return tx.Create(&stats).Error
-		} else if err != nil {
-			return err
-		}
-
-		return tx.Model(&stats).Update("request_count", gorm.Expr("request_count + 1")).Error
-	})
+	return r.db.Clauses(clause.OnConflict{
+		Columns: []clause.Column{
+			{Name: "provider_id"},
+			{Name: "model_name"},
+			{Name: "date"},
+		},
+		DoUpdates: clause.Assignments(map[string]interface{}{
+			"request_count": gorm.Expr("request_count + 1"),
+		}),
+	}).Create(&stats).Error
 }
 
 func (r *statsRepository) BatchUpdate(providerID, modelName string, date time.Time, delta int) error {
-	return r.db.Transaction(func(tx *gorm.DB) error {
-		var stats config.UsageStats
-		err := tx.Where("provider_id = ? AND model_name = ? AND date = ?",
-			providerID, modelName, date).First(&stats).Error
+	stats := config.UsageStats{
+		ProviderID:   providerID,
+		ModelName:    modelName,
+		RequestCount: delta,
+		Date:         date,
+	}
 
-		if errors.Is(err, gorm.ErrRecordNotFound) {
-			return tx.Create(&config.UsageStats{
-				ProviderID:   providerID,
-				ModelName:    modelName,
-				RequestCount: delta,
-				Date:         date,
-			}).Error
-		} else if err != nil {
-			return err
-		}
-
-		return tx.Model(&stats).
-			Update("request_count", gorm.Expr("request_count + ?", delta)).Error
-	})
+	return r.db.Clauses(clause.OnConflict{
+		Columns: []clause.Column{
+			{Name: "provider_id"},
+			{Name: "model_name"},
+			{Name: "date"},
+		},
+		DoUpdates: clause.Assignments(map[string]interface{}{
+			"request_count": gorm.Expr("request_count + ?", delta),
+		}),
+	}).Create(&stats).Error
 }
 
 func (r *statsRepository) Query(providerID, modelName string, startDate, endDate *time.Time) ([]domain.UsageStats, error) {