fix(media): 统一图片结果 URL 化并限制同步 Base64

将上游 URL 直接持久化，内联媒体经对象存储后仅保留 URL 与内部定位元数据；异步轮询、任务详情和幂等重放统一使用零对象读取的 URL 投影，并增加 64KiB 响应门禁。

OpenAI 图片接口接受 url 与 b64_json，同步 Base64 限制为 20MiB 和每 Pod 2 并发；新增历史结果迁移清零门禁、结果指标和 API GOMEMLIMIT。

验证：API go test ./...、go vet、聚焦 race、pnpm openapi、pnpm lint/test/build、迁移安全检查与 docker compose config 均通过。

apps/api/internal/httpapi/response.go

@@ -9,12 +9,43 @@ import (
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/publicerror"
 )
 
+const maxAsyncMediaResultResponseBytes = 64 << 10
+
 func writeJSON(w http.ResponseWriter, status int, value any) {
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.WriteHeader(status)
 	_ = json.NewEncoder(w).Encode(value)
 }
 
+func writeAsyncMediaResultJSON(w http.ResponseWriter, value any, observer interface {
+	ObserveResultDelivery(string, int64)
+}) {
+	payload, err := json.Marshal(value)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "encode task result failed", "internal_error")
+		return
+	}
+	if len(payload) > maxAsyncMediaResultResponseBytes {
+		if observer != nil {
+			observer.ObserveResultDelivery("poll_oversized", int64(len(payload)))
+		}
+		writeErrorWithDetails(w, http.StatusInternalServerError, "task result response exceeds the URL-only size limit", map[string]any{
+			"limit_bytes":  maxAsyncMediaResultResponseBytes,
+			"actual_bytes": len(payload),
+		}, "result_response_too_large")
+		return
+	}
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.Header().Set("X-Gateway-Response-Format", "url")
+	w.Header().Set("Deprecation", "true")
+	w.Header().Set("X-Gateway-Deprecated-Fields", "output_content,result")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(append(payload, '\n'))
+	if observer != nil {
+		observer.ObserveResultDelivery("poll_success", int64(len(payload)+1))
+	}
+}
+
 func writeError(w http.ResponseWriter, status int, message string, codes ...string) {
 	writeErrorWithDetails(w, status, message, nil, codes...)
 }