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/object_storage_test.go

@@ -2,6 +2,7 @@ package runner
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -371,10 +372,16 @@ func TestObjectStorageReadUsesChannelRetryPolicy(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	payload, err := readObjectStorageWithRetries(t.Context(), adapter, "media/request_asset/object.png")
+	var observed []string
+	payload, err := readObjectStorageWithRetriesObserved(t.Context(), adapter, "media/request_asset/object.png", func(event string, provider string, bytes int, _ time.Duration) {
+		observed = append(observed, fmt.Sprintf("%s:%s:%d", event, provider, bytes))
+	})
 	if err != nil || string(payload) != "stored-payload" || calls.Load() != 2 {
 		t.Fatalf("payload=%q calls=%d err=%v", payload, calls.Load(), err)
 	}
+	if got := strings.Join(observed, ","); got != "read_failure:s3:0,read_success:s3:14" {
+		t.Fatalf("observed=%s", got)
+	}
 }
 
 func TestObjectStorageAuthFailureSkipsSameChannelRetry(t *testing.T) {