feat(ai): 自由创作视频生成全量移植(/free-create)——Seedance 文/图生视频+按时长计费+人物素材库

后端:
- free_video.py: 提交/轮询/收藏/软删全链路,复用 AITask(新增 is_deleted/is_favorited, migration 0022)
- video_pricing.py: 按时长 token 计费(×1.10 buffer+clamp); video_errors.py 错误归一; media_probe.py 时长探测
- catalog/volcano: Seedance free-video 模型接入+seed(migration 0023)
- 素材库: FreeAssetGroup/FreeAsset(火山 Assets API 引用登记, migration 0009)+ 上传/轮询/删除接口
- settings: FREE_VIDEO_MAX_CONCURRENT 团队并发闸(默认3); CELERY_TASK_ALWAYS_EAGER 本地联调开关(生产恒关)
- 测试: test_free_video.py 新增; billing/products/projects tests 配套调整

前端:
- /free-create 页面+components/free-create/ 全套(输入栏/@mention 素材引用/生成卡/视频详情弹窗/素材库弹窗)
- api.ts/types.ts 扩展 free-video 与 free-assets 接口; 路由/侧边栏入口接入

bug/: 测试清单 (11)(12) 与截图归档

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/backend/apps/ai/services.py

@@ -698,6 +698,9 @@ def _store_generated_media(*, team, user, project, task, media: str, name: str,
         suffix = ".jpg"
     elif "webp" in content_type:
         suffix = ".webp"
+    # 先取字节再上传:boto3 upload_fileobj 完成后会 close 掉 BytesIO,之后 getvalue() 抛
+    # "I/O operation on closed file" 被下面的 except 吞掉 → 视频封面一直静默抽不出来(自由创作联调实测)。
+    raw_bytes = fileobj.getvalue() if isinstance(fileobj, BytesIO) else b""
     asset_id = uuid.uuid4()
     object_key = f"teams/{team.id}/projects/{project.id}/generated/{asset_id}{suffix}"
     stored = TosStorage().upload_fileobj(fileobj=fileobj, object_key=object_key, content_type=content_type)
@@ -721,11 +724,7 @@ def _store_generated_media(*, team, user, project, task, media: str, name: str,
     )
     # 视频资产:额外抽首帧作为封面图,挂成同一 Asset 下的 image 文件,供任务中心/列表显示缩略图
     if "video" in content_type:
-        try:
-            video_bytes = fileobj.getvalue() if isinstance(fileobj, BytesIO) else b""
-        except Exception:  # noqa: BLE001
-            video_bytes = b""
-        poster = _generate_video_poster(video_bytes=video_bytes, team=team, project=project, asset_id=asset_id)
+        poster = _generate_video_poster(video_bytes=raw_bytes, team=team, project=project, asset_id=asset_id)
         if poster:
             AssetFile.objects.create(
                 asset=asset,