fix(metrics): 补齐媒体结果存储读取与兼容响应指标

增加对象存储 GET、结果 URL 签名和同步 Base64 限流指标，使生产轮询可直接验证零对象下载，并区分签名失败、容量等待超时和 20MiB 超限。\n\n验证：API 全量 go test、go vet、gofmt 与 git diff --check。

apps/api/internal/runner/request_assets.go

@@ -292,7 +292,7 @@ func (s *Service) readRequestAssetBytes(ctx context.Context, asset store.Request
 		if err != nil {
 			return nil, err
 		}
-		return readObjectStorageWithRetries(ctx, adapter, asset.ObjectKey)
+		return readObjectStorageWithRetriesObserved(ctx, adapter, asset.ObjectKey, s.observeObjectStorage)
 	}
 	if strings.TrimSpace(asset.LocalPath) != "" {
 		payload, err := os.ReadFile(asset.LocalPath)
@@ -331,10 +331,22 @@ func (s *Service) readRequestAssetBytes(ctx context.Context, asset store.Request
 }
 
 func readObjectStorageWithRetries(ctx context.Context, adapter *objectStorageAdapter, objectKey string) ([]byte, error) {
+	return readObjectStorageWithRetriesObserved(ctx, adapter, objectKey, nil)
+}
+
+func readObjectStorageWithRetriesObserved(ctx context.Context, adapter *objectStorageAdapter, objectKey string, observe func(string, string, int, time.Duration)) ([]byte, error) {
 	maxRetries, delays := uploadRetrySchedule(adapter.channel.RetryPolicy, adapter.channel.Provider)
 	var lastErr error
 	for attempt := 0; attempt <= maxRetries; attempt++ {
+		startedAt := time.Now()
 		payload, err := adapter.get(ctx, objectKey)
+		if observe != nil {
+			event := "read_success"
+			if err != nil {
+				event = "read_failure"
+			}
+			observe(event, adapter.channel.Provider, len(payload), time.Since(startedAt))
+		}
 		if err == nil {
 			return payload, nil
 		}
@@ -366,7 +378,13 @@ func (s *Service) requestAssetAccessURL(ctx context.Context, asset store.Request
 			return value, nil
 		}
 	}
-	return adapter.presignGet(asset.ObjectKey, objectStorageSignedURLTTL)
+	value, err := adapter.presignGet(asset.ObjectKey, objectStorageSignedURLTTL)
+	if err != nil {
+		s.observeResultURLSigning("failure")
+		return "", err
+	}
+	s.observeResultURLSigning("success")
+	return value, nil
 }
 
 func (s *Service) fileStorageChannelForAsset(ctx context.Context, asset store.RequestAsset) (store.FileStorageChannel, error) {