fix(P0): 素材库视频上传时生成缩略图

- ingest流程中添加视频缩略图生成（使用15%处抽帧）
- 缩略图生成失败不影响主流程，仅打warning日志
- 修复素材库视频全部显示渐变占位图的问题

Issue: #577

apps/worker/worker_app/tasks/ingest.py

@@ -280,6 +280,7 @@ def ingest_asset(job_id: str) -> dict:
         # 先从 OSS 下载文件到本地临时目录，再提取元数据
         # （storage_key 是 OSS 内部路径，不能直接传给 ffprobe/Pillow）
         local_file = None
+        thumbnail_url = None
         try:
             suffix = Path(job.storage_key).suffix or ".bin"
             with tempfile.NamedTemporaryFile(suffix=suffix, delete=False) as tmp:
@@ -291,6 +292,29 @@ def ingest_asset(job_id: str) -> dict:
                 metadata, extract_success = {}, False
             else:
                 metadata, extract_success = extract_media_metadata(str(local_file), media_type)
+
+            # 视频类型：生成缩略图（文件还在的时候生成）
+            thumbnail_url = None
+            if media_type == "video" and extract_success:
+                try:
+                    from video_processing.thumbnail_generator import generate_and_upload_thumbnail
+
+                    thumb_storage_key = f"assets/{job.project_id}/thumbnails/{job_id}.jpg"
+                    thumbnail_url = generate_and_upload_thumbnail(
+                        str(local_file), thumb_storage_key
+                    )
+                    if thumbnail_url:
+                        logger.info(
+                            "素材缩略图生成成功: job_id=%s url=%s",
+                            job_id,
+                            thumbnail_url[:80],
+                        )
+                except Exception as thumb_err:
+                    logger.warning(
+                        "素材缩略图生成失败（不影响主流程）: job_id=%s error=%s",
+                        job_id,
+                        thumb_err,
+                    )
         finally:
             if local_file and local_file.exists():
                 try:
@@ -359,6 +383,7 @@ def ingest_asset(job_id: str) -> dict:
             codec=metadata.get("codec") or None,
             status=AssetStatus.READY,
             file_hash=job.file_hash,
+            thumbnail_url=thumbnail_url,
         )
         asset_repo.create(asset)