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/frontend/src/App.tsx

@@ -24,6 +24,7 @@ import {
   AssetFactoryPage,
   AuthScreen,
   Dashboard,
+  FreeCreatePage,
   ImageWorkbenchPage,
   LibraryPage,
   MessagesPage,
@@ -878,6 +879,8 @@ export function App() {
         );
       case "assetFactory":
         return <AssetFactoryPage navigate={navigate} />;
+      case "freeCreate":
+        return <FreeCreatePage modelConfigs={modelConfigs} onNotify={(type, text) => setNotice({ type, text })} />;
       case "imageOptimize":
         return <ImageWorkbenchPage mode="image" products={products} modelConfigs={modelConfigs} initialProductId={activeProductId} onProductChange={setActiveProductId} unreadByProduct={aiUnreadByProduct} onProductViewed={markAiRead} onBack={() => navigate("assetFactory")} navigate={navigate} onGenerate={generateImages} onResume={resumeImages} />;
       case "modelPhoto":