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.json

@@ -5656,6 +5656,16 @@
                         "description": "OK",
                         "schema": {
                             "$ref": "#/definitions/httpapi.EasyAIGeneratedResponse"
+                        },
+                        "headers": {
+                            "Deprecation": {
+                                "type": "string",
+                                "description": "output_content 与 result 兼容别名的废弃标记"
+                            },
+                            "X-Gateway-Response-Format": {
+                                "type": "string",
+                                "description": "实际结果格式，异步轮询固定为 url"
+                            }
                         }
                     },
                     "404": {
@@ -11167,6 +11177,7 @@
                     }
                 },
                 "output_content": {
+                    "description": "OutputContent is a one-release URL-only compatibility alias for Data.",
                     "type": "array",
                     "items": {
                         "$ref": "#/definitions/httpapi.EasyAIMediaOutput"
@@ -11177,6 +11188,7 @@
                     "example": "/api/v1/ai/result/9f4d8f3d-5f5f-4bb7-a4be-344a9f930e25"
                 },
                 "result": {
+                    "description": "Result is a one-release URL-only compatibility alias and is deprecated.",
                     "type": "object",
                     "additionalProperties": true
                 },
@@ -11218,7 +11230,7 @@
                     "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": {