refactor: 统一封面生成管道 — 从渲染后视频抽帧作为封面

核心改动：
- 渲染完成后从已渲染视频抽帧作为封面，天然带标题（ASS字幕已烧录）
- 删除3条旧封面路径：缩略图、候选帧、FFmpeg备用抽帧
- 封面URL持久化到GenerationTask.cover_url
- 封面API直接从持久化URL读取

apps/worker/video_processing/dedup_helpers.py

@@ -85,19 +85,9 @@ def create_video_record_and_dedup(
         if thumbnail_url:
             generated_video.thumbnail_url = thumbnail_url
             video_repo.update_thumbnail(video_id, thumbnail_url)
-            logger.info("Thumbnail reused (pre-generated) for video %s", video_id)
+            logger.info("Thumbnail set for video %s: %s", video_id, thumbnail_url[:80] if thumbnail_url else "")
         else:
-            thumbnail_storage_key = f"generated/projects/{project_id}/thumbnails/{video_id}.jpg"
-            try:
-                from video_processing.thumbnail_generator import generate_and_upload_thumbnail
-
-                _thumbnail_url = generate_and_upload_thumbnail(video_path, thumbnail_storage_key)
-                if _thumbnail_url:
-                    generated_video.thumbnail_url = _thumbnail_url
-                    video_repo.update_thumbnail(video_id, _thumbnail_url)
-                    logger.info("Thumbnail generated for video %s: %s", video_id, _thumbnail_url)
-            except Exception as thumb_err:
-                logger.warning("Thumbnail generation failed for %s: %s", video_id, thumb_err)
+            logger.debug("No thumbnail_url provided for video %s, skipping", video_id)
 
         # 计算视频指纹
         deduplicator = VideoDeduplicator()