feat(P1): 素材列表支持按类型过滤 - 视频库/配音库分类

新增/修复:
- Asset domain 增加 file_type property（从 mime_type 推导）
- AssetRepository 新增 find_by_project_and_file_type / count_by_project_and_file_type / count_by_library_and_file_type
- 素材列表接口 kind 过滤全路径覆盖（DB层分页更高效）：
  - library_id模式：total改用count查询（之前用len(items)不准）
  - project_id模式：改用DB层file_type查询（之前内存过滤性能差）
  - 跨项目模式：新增kind过滤（之前完全没有）
  - 内存过滤路径：project_id和跨项目增加DB层预过滤+改用file_type判断
- 向后兼容：kind参数已存在，只是之前不完整

packages/adapters/sqlalchemy_impl/asset_repository.py

@@ -53,6 +53,48 @@ class SQLAlchemyAssetRepository:
         models = query.order_by(AssetModel.created_at.desc()).offset(skip).limit(limit).all()
         return [self._to_domain(model) for model in models]
 
+    def count_by_library_and_file_type(
+        self,
+        library_id: str,
+        file_type: str,
+        status: list[str] | None = None,
+    ) -> int:
+        query = self.session.query(AssetModel).filter(
+            AssetModel.asset_library_id == library_id, AssetModel.file_type == file_type
+        )
+        if status:
+            query = query.filter(AssetModel.status.in_(status))
+        return query.count()
+
+    def find_by_project_and_file_type(
+        self,
+        project_id: str,
+        file_type: str,
+        skip: int = 0,
+        limit: int = 100,
+        status: list[str] | None = None,
+    ) -> list[Asset]:
+        query = self.session.query(AssetModel).filter(
+            AssetModel.project_id == project_id, AssetModel.file_type == file_type
+        )
+        if status:
+            query = query.filter(AssetModel.status.in_(status))
+        models = query.order_by(AssetModel.created_at.desc()).offset(skip).limit(limit).all()
+        return [self._to_domain(model) for model in models]
+
+    def count_by_project_and_file_type(
+        self,
+        project_id: str,
+        file_type: str,
+        status: list[str] | None = None,
+    ) -> int:
+        query = self.session.query(AssetModel).filter(
+            AssetModel.project_id == project_id, AssetModel.file_type == file_type
+        )
+        if status:
+            query = query.filter(AssetModel.status.in_(status))
+        return query.count()
+
     def find_by_id(self, asset_id: str) -> Asset | None:
         model = self.session.query(AssetModel).filter(AssetModel.id == asset_id).first()
         if model is None: