feat(api): add API key stats flush and summary endpoints

Introduce internal endpoint for flushing accumulated APIKey statistics
from data plane to control plane database, updating both individual
API keys and their parent provider groups with request counts and
success/failure rates.

Add admin endpoint to retrieve aggregated API key statistics summary
across all provider groups, including total requests, success/failure
counts, and calculated rates.

internal/api/apikey_stats_handler.go

@@ -0,0 +1,67 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/ez-api/ez-api/internal/model"
+	"github.com/gin-gonic/gin"
+)
+
+type APIKeyStatsSummaryResponse struct {
+	TotalRequests   int64   `json:"total_requests"`
+	SuccessRequests int64   `json:"success_requests"`
+	FailureRequests int64   `json:"failure_requests"`
+	SuccessRate     float64 `json:"success_rate"`
+	FailureRate     float64 `json:"failure_rate"`
+}
+
+// GetAPIKeyStatsSummary godoc
+// @Summary      APIKey stats summary (admin)
+// @Description  Aggregate APIKey success/failure stats across all provider groups
+// @Tags         admin
+// @Produce      json
+// @Security     AdminAuth
+// @Success      200  {object}  APIKeyStatsSummaryResponse
+// @Failure      500  {object}  gin.H
+// @Router       /admin/apikey-stats/summary [get]
+func (h *AdminHandler) GetAPIKeyStatsSummary(c *gin.Context) {
+	if h == nil || h.db == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "database not configured"})
+		return
+	}
+
+	var totals struct {
+		TotalRequests   int64 `json:"total_requests"`
+		SuccessRequests int64 `json:"success_requests"`
+		FailureRequests int64 `json:"failure_requests"`
+	}
+
+	if err := h.db.Model(&model.APIKey{}).
+		Select("COALESCE(SUM(total_requests),0) as total_requests, COALESCE(SUM(success_requests),0) as success_requests").
+		Scan(&totals).Error; err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to aggregate api key stats", "details": err.Error()})
+		return
+	}
+
+	total := totals.TotalRequests
+	success := totals.SuccessRequests
+	failure := total - success
+	if failure < 0 {
+		failure = 0
+	}
+
+	var successRate float64
+	var failureRate float64
+	if total > 0 {
+		successRate = float64(success) / float64(total)
+		failureRate = float64(failure) / float64(total)
+	}
+
+	c.JSON(http.StatusOK, APIKeyStatsSummaryResponse{
+		TotalRequests:   total,
+		SuccessRequests: success,
+		FailureRequests: failure,
+		SuccessRate:     successRate,
+		FailureRate:     failureRate,
+	})
+}