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/handler/stats_handler.go

@@ -6,20 +6,21 @@ import (
 
 	"github.com/gin-gonic/gin"
 
-	"nex/backend/internal/config"
+	"nex/backend/internal/service"
 )
 
 // StatsHandler 统计处理器
-type StatsHandler struct{}
+type StatsHandler struct {
+	statsService service.StatsService
+}
 
 // NewStatsHandler 创建统计处理器
-func NewStatsHandler() *StatsHandler {
-	return &StatsHandler{}
+func NewStatsHandler(statsService service.StatsService) *StatsHandler {
+	return &StatsHandler{statsService: statsService}
 }
 
 // GetStats 查询统计
 func (h *StatsHandler) GetStats(c *gin.Context) {
-	// 解析查询参数
 	providerID := c.Query("provider_id")
 	modelName := c.Query("model_name")
 	startDateStr := c.Query("start_date")
@@ -27,7 +28,6 @@ func (h *StatsHandler) GetStats(c *gin.Context) {
 
 	var startDate, endDate *time.Time
 
-	// 解析日期
 	if startDateStr != "" {
 		t, err := time.Parse("2006-01-02", startDateStr)
 		if err != nil {
@@ -50,8 +50,7 @@ func (h *StatsHandler) GetStats(c *gin.Context) {
 		endDate = &t
 	}
 
-	// 查询统计
-	stats, err := config.GetStats(providerID, modelName, startDate, endDate)
+	stats, err := h.statsService.Get(providerID, modelName, startDate, endDate)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{
 			"error": "查询统计失败: " + err.Error(),
@@ -64,16 +63,14 @@ func (h *StatsHandler) GetStats(c *gin.Context) {
 
 // AggregateStats 聚合统计
 func (h *StatsHandler) AggregateStats(c *gin.Context) {
-	// 解析查询参数
 	providerID := c.Query("provider_id")
 	modelName := c.Query("model_name")
 	startDateStr := c.Query("start_date")
 	endDateStr := c.Query("end_date")
-	groupBy := c.Query("group_by") // "provider", "model", "date"
+	groupBy := c.Query("group_by")
 
 	var startDate, endDate *time.Time
 
-	// 解析日期
 	if startDateStr != "" {
 		t, err := time.Parse("2006-01-02", startDateStr)
 		if err != nil {
@@ -96,8 +93,7 @@ func (h *StatsHandler) AggregateStats(c *gin.Context) {
 		endDate = &t
 	}
 
-	// 查询统计
-	stats, err := config.GetStats(providerID, modelName, startDate, endDate)
+	stats, err := h.statsService.Get(providerID, modelName, startDate, endDate)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{
 			"error": "查询统计失败: " + err.Error(),
@@ -105,80 +101,6 @@ func (h *StatsHandler) AggregateStats(c *gin.Context) {
 		return
 	}
 
-	// 聚合
-	result := h.aggregate(stats, groupBy)
-
+	result := h.statsService.Aggregate(stats, groupBy)
 	c.JSON(http.StatusOK, result)
 }
-
-// aggregate 执行聚合
-func (h *StatsHandler) aggregate(stats []config.UsageStats, groupBy string) []map[string]interface{} {
-	switch groupBy {
-	case "provider":
-		return h.aggregateByProvider(stats)
-	case "model":
-		return h.aggregateByModel(stats)
-	case "date":
-		return h.aggregateByDate(stats)
-	default:
-		// 默认按供应商聚合
-		return h.aggregateByProvider(stats)
-	}
-}
-
-// aggregateByProvider 按供应商聚合
-func (h *StatsHandler) aggregateByProvider(stats []config.UsageStats) []map[string]interface{} {
-	aggregated := make(map[string]int)
-	for _, stat := range stats {
-		aggregated[stat.ProviderID] += stat.RequestCount
-	}
-
-	result := make([]map[string]interface{}, 0, len(aggregated))
-	for providerID, count := range aggregated {
-		result = append(result, map[string]interface{}{
-			"provider_id":   providerID,
-			"request_count": count,
-		})
-	}
-
-	return result
-}
-
-// aggregateByModel 按模型聚合
-func (h *StatsHandler) aggregateByModel(stats []config.UsageStats) []map[string]interface{} {
-	aggregated := make(map[string]int)
-	for _, stat := range stats {
-		key := stat.ProviderID + "/" + stat.ModelName
-		aggregated[key] += stat.RequestCount
-	}
-
-	result := make([]map[string]interface{}, 0, len(aggregated))
-	for key, count := range aggregated {
-		result = append(result, map[string]interface{}{
-			"provider_id":   key[:len(key)/2],
-			"model_name":    key[len(key)/2+1:],
-			"request_count": count,
-		})
-	}
-
-	return result
-}
-
-// aggregateByDate 按日期聚合
-func (h *StatsHandler) aggregateByDate(stats []config.UsageStats) []map[string]interface{} {
-	aggregated := make(map[string]int)
-	for _, stat := range stats {
-		key := stat.Date.Format("2006-01-02")
-		aggregated[key] += stat.RequestCount
-	}
-
-	result := make([]map[string]interface{}, 0, len(aggregated))
-	for date, count := range aggregated {
-		result = append(result, map[string]interface{}{
-			"date":          date,
-			"request_count": count,
-		})
-	}
-
-	return result
-}