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.

sdk/cliproxy/auth/types.go

@@ -164,7 +164,7 @@ func recentRequestBucketIndex(bucketID int64) int {
 
 func formatRecentRequestBucketLabel(bucketID int64) string {
 	start := time.Unix(bucketID*recentRequestBucketSeconds, 0).In(time.Local)
-	end := start.Add(10 * time.Minute)
+	end := start.Add(time.Duration(recentRequestBucketSeconds) * time.Second)
 	return start.Format("15:04") + "-" + end.Format("15:04")
 }