feat: 初始化 AI Gateway 项目

实现支持 OpenAI 和 Anthropic 双协议的统一大模型 API 网关 MVP 版本，包含：
- OpenAI 和 Anthropic 协议代理
- 供应商和模型管理
- 用量统计
- 前端配置界面

backend/internal/handler/stats_handler.go

@@ -0,0 +1,184 @@
+package handler
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"nex/backend/internal/config"
+)
+
+// StatsHandler 统计处理器
+type StatsHandler struct{}
+
+// NewStatsHandler 创建统计处理器
+func NewStatsHandler() *StatsHandler {
+	return &StatsHandler{}
+}
+
+// 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")
+	endDateStr := c.Query("end_date")
+
+	var startDate, endDate *time.Time
+
+	// 解析日期
+	if startDateStr != "" {
+		t, err := time.Parse("2006-01-02", startDateStr)
+		if err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": "无效的 start_date 格式，应为 YYYY-MM-DD",
+			})
+			return
+		}
+		startDate = &t
+	}
+
+	if endDateStr != "" {
+		t, err := time.Parse("2006-01-02", endDateStr)
+		if err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": "无效的 end_date 格式，应为 YYYY-MM-DD",
+			})
+			return
+		}
+		endDate = &t
+	}
+
+	// 查询统计
+	stats, err := config.GetStats(providerID, modelName, startDate, endDate)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{
+			"error": "查询统计失败: " + err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, stats)
+}
+
+// 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"
+
+	var startDate, endDate *time.Time
+
+	// 解析日期
+	if startDateStr != "" {
+		t, err := time.Parse("2006-01-02", startDateStr)
+		if err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": "无效的 start_date 格式，应为 YYYY-MM-DD",
+			})
+			return
+		}
+		startDate = &t
+	}
+
+	if endDateStr != "" {
+		t, err := time.Parse("2006-01-02", endDateStr)
+		if err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": "无效的 end_date 格式，应为 YYYY-MM-DD",
+			})
+			return
+		}
+		endDate = &t
+	}
+
+	// 查询统计
+	stats, err := config.GetStats(providerID, modelName, startDate, endDate)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{
+			"error": "查询统计失败: " + err.Error(),
+		})
+		return
+	}
+
+	// 聚合
+	result := h.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
+}