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/docs/swagger.yaml

@@ -409,6 +409,8 @@ definitions:
           type: string
         type: array
       output_content:
+        description: OutputContent is a one-release URL-only compatibility alias for
+          Data.
         items:
           $ref: '#/definitions/httpapi.EasyAIMediaOutput'
         type: array
@@ -417,6 +419,7 @@ definitions:
         type: string
       result:
         additionalProperties: true
+        description: Result is a one-release URL-only compatibility alias and is deprecated.
         type: object
       status:
         enum:
@@ -446,7 +449,8 @@ definitions:
       audio_url:
         type: string
       b64_json:
-        description: B64JSON is retained when result media transfer is disabled.
+        description: B64JSON is available only to bounded synchronous OpenAI image
+          responses.
         type: string
       content:
         type: string
@@ -8093,6 +8097,13 @@ paths:
       responses:
         "200":
           description: OK
+          headers:
+            Deprecation:
+              description: output_content 与 result 兼容别名的废弃标记
+              type: string
+            X-Gateway-Response-Format:
+              description: 实际结果格式，异步轮询固定为 url
+              type: string
           schema:
             $ref: '#/definitions/httpapi.EasyAIGeneratedResponse'
         "404":