feat: 新增 POST /assets/batch 批量获取素材详情接口

前端 usePreviewAssets.ts 调用 POST /assets/batch 时返回 405 Method Not Allowed，
原因是后端没有 /batch GET/POST 端点，请求命中了 GET /{asset_id}（batch 被当成 asset_id）。

新增：
- BatchGetRequest schema：接收 { ids: string[] }，最多 200 个 ID
- POST /assets/batch 端点：调用 asset_repository.find_by_ids() 批量查询
- 返回 List[AssetResponse]，包含 file_url、duration、width、height 等完整元数据

改动文件：
- apps/api/app/schemas/asset.py：新增 BatchGetRequest
- apps/api/app/api/routes/assets.py：新增 batch_get_assets 端点

apps/api/app/api/routes/assets.py

@@ -1,5 +1,5 @@
 import logging
-from typing import Any, Optional
+from typing import Any, List, Optional
 
 from app.api.routes._helpers import check_project_access, format_utc_datetime
 from app.auth import AuthenticatedUser, get_current_user
@@ -14,6 +14,7 @@ from app.schemas.asset import (
     AssetResponse,
     BatchClassifyRequest,
     BatchDeleteRequest,
+    BatchGetRequest,
     BatchMarkRequest,
     BatchOperationResponse,
     BatchTagRequest,
@@ -370,6 +371,18 @@ def update_asset_review_status(
     return _to_asset_response(updated)
 
 
+@router.post("/batch", response_model=List[AssetResponse])
+def batch_get_assets(
+    request: BatchGetRequest,
+    authenticated_user: AuthenticatedUser = Depends(get_current_user),
+    asset_repository: Any = Depends(get_asset_repository),
+) -> list[AssetResponse]:
+    """批量获取素材详情（根据 ID 列表）。"""
+    items = asset_repository.find_by_ids(request.ids)
+    storage_service = get_storage_service()
+    return [_to_asset_response(item, storage_service) for item in items]
+
+
 @router.post("/batch-delete", response_model=BatchOperationResponse)
 def batch_delete_assets(
     request: BatchDeleteRequest,

apps/api/app/schemas/asset.py

@@ -58,6 +58,12 @@ class AssetResponse(BaseModel):
 MAX_BATCH_SIZE = 200
 
 
+class BatchGetRequest(BaseModel):
+    """批量获取素材详情请求。"""
+
+    ids: list[str] = Field(..., min_length=1, max_length=MAX_BATCH_SIZE, description="素材 ID 列表")
+
+
 class BatchDeleteRequest(BaseModel):
     """批量删除请求（软删除）。"""