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/assets/models.py

@@ -178,3 +178,61 @@ class AssetUsage(TimeStampedModel):
     usage_type = models.CharField(max_length=64)
     context = models.JSONField(default=dict, blank=True)
 
+
+class FreeAssetGroup(TeamOwnedModel):
+    """自由创作·人物素材组(一角色一组)。
+
+    本质是「火山 Assets API 引用登记表」:组内素材登记到火山素材库拿 remote_asset_id,
+    生成时以 asset://{remote_asset_id} 引用(免重复上传 + 火山侧预审)。
+    与 AssetReviewGroup(一团队一审核组)职责不同,与资产库 Asset(TOS 文件资产)也不同,独立建表。
+    """
+
+    name = models.CharField(max_length=128)
+    description = models.TextField(blank=True)
+    remote_group_id = models.CharField(max_length=128)
+    thumbnail_url = models.URLField(blank=True)
+    is_deleted = models.BooleanField(default=False)
+
+    class Meta:
+        indexes = [
+            models.Index(fields=["team", "-created_at"]),
+        ]
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return f"free-group:{self.name}"
+
+
+class FreeAsset(TimeStampedModel):
+    """自由创作·素材组内单素材(火山 Assets API 登记项)。"""
+
+    class Type(models.TextChoices):
+        IMAGE = "Image", "Image"
+        VIDEO = "Video", "Video"
+        AUDIO = "Audio", "Audio"
+
+    class Status(models.TextChoices):
+        PROCESSING = "processing", "Processing"  # 火山侧处理/审核中
+        ACTIVE = "active", "Active"  # 可用,可 asset:// 引用
+        FAILED = "failed", "Failed"  # 审核/处理失败
+
+    group = models.ForeignKey(FreeAssetGroup, on_delete=models.CASCADE, related_name="assets")
+    remote_asset_id = models.CharField(max_length=128)
+    name = models.CharField(max_length=128, blank=True)
+    url = models.URLField(max_length=1024, blank=True)  # TOS 公读直链(火山从这里拉源文件)
+    asset_type = models.CharField(max_length=16, choices=Type.choices, default=Type.IMAGE)
+    thumbnail_url = models.URLField(max_length=1024, blank=True)
+    duration = models.FloatField(null=True, blank=True)  # 视频/音频时长(秒)
+    status = models.CharField(max_length=16, choices=Status.choices, default=Status.PROCESSING)
+    error_message = models.TextField(blank=True)
+
+    class Meta:
+        indexes = [
+            models.Index(fields=["group", "-created_at"]),
+            models.Index(fields=["remote_asset_id"]),
+        ]
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return f"free-asset:{self.name or self.remote_asset_id}"
+