feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/internal/repository/stats_repo_impl.go

@@ -0,0 +1,79 @@
+package repository
+
+import (
+	"errors"
+	"time"
+
+	"gorm.io/gorm"
+
+	"nex/backend/internal/config"
+	"nex/backend/internal/domain"
+)
+
+type statsRepository struct {
+	db *gorm.DB
+}
+
+func NewStatsRepository(db *gorm.DB) StatsRepository {
+	return &statsRepository{db: db}
+}
+
+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
+
+		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
+	})
+}
+
+func (r *statsRepository) Query(providerID, modelName string, startDate, endDate *time.Time) ([]domain.UsageStats, error) {
+	var stats []config.UsageStats
+	query := r.db.Model(&config.UsageStats{})
+
+	if providerID != "" {
+		query = query.Where("provider_id = ?", providerID)
+	}
+	if modelName != "" {
+		query = query.Where("model_name = ?", modelName)
+	}
+	if startDate != nil {
+		query = query.Where("date >= ?", startDate)
+	}
+	if endDate != nil {
+		query = query.Where("date <= ?", endDate)
+	}
+
+	err := query.Order("date DESC").Find(&stats).Error
+	if err != nil {
+		return nil, err
+	}
+
+	result := make([]domain.UsageStats, len(stats))
+	for i := range stats {
+		result[i] = domain.UsageStats{
+			ID:           stats[i].ID,
+			ProviderID:   stats[i].ProviderID,
+			ModelName:    stats[i].ModelName,
+			RequestCount: stats[i].RequestCount,
+			Date:         stats[i].Date,
+		}
+	}
+	return result, nil
+}