feat: add API key usage endpoint with provider and key grouping

- Implemented `GetAPIKeyUsage` to expose recent request data grouped by provider and API key.
- Added supporting function `mergeRecentRequestBuckets` for bucket aggregation.
- Registered new endpoint `/v0/management/api-key-usage` in the management API.
- Included extensive unit tests for provider and key-based grouping validation.
- Updated `formatRecentRequestBucketLabel` to support configurable bucket duration.

internal/api/handlers/management/api_key_usage.go

@@ -0,0 +1,86 @@
+package management
+
+import (
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
+)
+
+func mergeRecentRequestBuckets(dst, src []coreauth.RecentRequestBucket) []coreauth.RecentRequestBucket {
+	if len(dst) == 0 {
+		return src
+	}
+	if len(src) == 0 {
+		return dst
+	}
+	if len(dst) != len(src) {
+		n := len(dst)
+		if len(src) < n {
+			n = len(src)
+		}
+		for i := 0; i < n; i++ {
+			dst[i].Success += src[i].Success
+			dst[i].Failed += src[i].Failed
+		}
+		return dst
+	}
+	for i := range dst {
+		dst[i].Success += src[i].Success
+		dst[i].Failed += src[i].Failed
+	}
+	return dst
+}
+
+// GetAPIKeyUsage returns recent request buckets for all in-memory api_key auths,
+// grouped by provider and keyed by the raw api-key value.
+func (h *Handler) GetAPIKeyUsage(c *gin.Context) {
+	if h == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "handler not initialized"})
+		return
+	}
+
+	h.mu.Lock()
+	manager := h.authManager
+	h.mu.Unlock()
+	if manager == nil {
+		c.JSON(http.StatusServiceUnavailable, gin.H{"error": "core auth manager unavailable"})
+		return
+	}
+
+	now := time.Now()
+	out := make(map[string]map[string][]coreauth.RecentRequestBucket)
+	for _, auth := range manager.List() {
+		if auth == nil {
+			continue
+		}
+		kind, apiKey := auth.AccountInfo()
+		if !strings.EqualFold(strings.TrimSpace(kind), "api_key") {
+			continue
+		}
+		apiKey = strings.TrimSpace(apiKey)
+		if apiKey == "" {
+			continue
+		}
+		provider := strings.ToLower(strings.TrimSpace(auth.Provider))
+		if provider == "" {
+			provider = "unknown"
+		}
+
+		recent := auth.RecentRequestsSnapshot(now)
+		providerBucket, ok := out[provider]
+		if !ok {
+			providerBucket = make(map[string][]coreauth.RecentRequestBucket)
+			out[provider] = providerBucket
+		}
+		if existing, exists := providerBucket[apiKey]; exists {
+			providerBucket[apiKey] = mergeRecentRequestBuckets(existing, recent)
+			continue
+		}
+		providerBucket[apiKey] = recent
+	}
+
+	c.JSON(http.StatusOK, out)
+}