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/billing/tests.py

@@ -15,7 +15,7 @@ class CreditLedgerTests(TestCase):
         self.team = Team.objects.create(name="Billing Team", owner=self.user)
         TeamMember.objects.create(team=self.team, user=self.user, role=TeamMember.Role.OWNER)
         self.account = CreditAccount.objects.create(team=self.team, balance=Decimal("100.0000"))
-        self.provider = ModelProvider.objects.create(name="volcengine", display_name="Volcano")
+        self.provider, _ = ModelProvider.objects.get_or_create(name="volcengine", defaults={"display_name": "Volcano"})
         self.model = ModelConfig.objects.create(
             provider=self.provider,
             name="doubao-seed-2-0-pro-260215",
@@ -110,7 +110,7 @@ class MemberLimitTests(TestCase):
         self.team = Team.objects.create(name="Limit Team", owner=self.user)
         self.member = TeamMember.objects.create(team=self.team, user=self.user, role=TeamMember.Role.OWNER)
         CreditAccount.objects.create(team=self.team, balance=Decimal("1000.0000"))
-        self.provider = ModelProvider.objects.create(name="volcengine", display_name="Volcano")
+        self.provider, _ = ModelProvider.objects.get_or_create(name="volcengine", defaults={"display_name": "Volcano"})
         self.model = ModelConfig.objects.create(
             provider=self.provider, name="m", display_name="M", capability=ModelConfig.Capability.IMAGE,
         )