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/components/app-shell.tsx

@@ -17,6 +17,7 @@ const SHELL_COMMANDS: Command[] = [
   { id: "products", group: "导航", label: "商品库", sub: "管理 SKU、商品图册、卖点信息", page: "products", icon: "package", key: "P" },
   { id: "projects", group: "导航", label: "视频项目", sub: "查看五阶段短视频流水线", page: "projects", icon: "clapperboard", key: "V" },
   { id: "asset-factory", group: "导航", label: "图片生成", sub: "模特上身图、平台套图、图片创作", page: "assetFactory", icon: "sparkles", key: "I" },
+  { id: "free-create", group: "导航", label: "自由创作", sub: "AI 视频生成 · 全能参考 / 首尾帧", page: "freeCreate", icon: "film", key: "F" },
   { id: "library", group: "导航", label: "资产库", sub: "素材、人物、场景、成片统一管理", page: "library", icon: "folder", key: "A" },
   { id: "team", group: "导航", label: "团队", sub: "成员、权限、额度、协作记录", page: "team", icon: "users" },
   { id: "account", group: "导航", label: "消费", sub: "余额、充值、账单流水", page: "account", icon: "creditCard" },
@@ -197,6 +198,7 @@ const NAV: NavDef[] = [
   { id: "models", page: "models", label: "模特库", icon: "model" },
   { id: "projects", page: "projects", label: "视频项目", icon: "clapperboard" },
   { id: "asset-factory", page: "assetFactory", label: "图片生成", icon: "sparkles" },
+  { id: "free-create", page: "freeCreate", label: "自由创作", icon: "film" },
   { id: "library", page: "library", label: "资产库", icon: "library" },
   { id: "team", page: "team", label: "团队", icon: "users" },
   { id: "account", page: "account", label: "消费", icon: "creditCard" },
@@ -219,6 +221,7 @@ const PAGE_TO_NAV: Partial<Record<Page, Page>> = {
   modelPhotoDemoA: "assetFactory",
   modelPhotoDemoB: "assetFactory",
   platformCover: "assetFactory",
+  freeCreate: "freeCreate",
   library: "library",
   team: "team",
   account: "account",