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/worker_app/tasks/generation.py

@@ -1124,14 +1124,14 @@ def _render_video(
     bgm_config: dict | None = None,
     voice_ids: list[str] | None = None,
     custom_title: str = "",
-) -> tuple[Path, float]:
+) -> tuple[Path, float, str]:
     """渲染视频（含配音混音）。
 
-    使用 RenderAdapter 统一渲染入口，复用 BGM/ASR/分辨率/缩略图逻辑。
+    使用 RenderAdapter 统一渲染入口，复用 BGM/ASR/分辨率/封面抽取逻辑。
 
     Args:
     Returns:
-        (output_path, render_duration)
+        (output_path, render_duration, cover_url)
     """
     if not downloaded_videos:
         raise RuntimeError(f"素材下载结果为空: task_id={task_id}")
@@ -1259,8 +1259,9 @@ def _render_video(
 
     # 配音素材库音频已在统一渲染引擎内部通过 audio 图层混音处理
     output_path = render_output_path
+    cover_url = getattr(render_result, "cover_url", "") or ""
 
-    return output_path, render_duration
+    return output_path, render_duration, cover_url
 
 
 def _upload_and_record(
@@ -1271,6 +1272,7 @@ def _upload_and_record(
     editing_mode,
     user_id: str = "",
     video_name: str = "",
+    thumbnail_url: str = "",
 ) -> tuple[str, float, int, int]:
     """上传 OSS、创建视频记录并查重。
 
@@ -1531,7 +1533,7 @@ def generate_video(self, task_id: str) -> dict:
             else:
                 _resolved_resolution = task_info.get("resolution", "")
 
-            output_path, render_duration = _render_video(
+            output_path, render_duration, cover_url = _render_video(
                 task_id=task_id,
                 downloaded_videos=downloaded_videos,
                 voice_path=audio_path,
@@ -1551,6 +1553,32 @@ def generate_video(self, task_id: str) -> dict:
                 gen_task.append_log("渲染", f"渲染完成, 时长={render_duration:.1f}s")
                 _flush_logs(task_id, gen_task)
 
+            # 持久化封面 URL 到 GenerationTask（统一封面管道：从渲染后视频抽帧）
+            if cover_url:
+                try:
+                    _cover_session = SessionLocal()
+                    try:
+                        from packages.adapters.sqlalchemy_impl.models import GenerationTaskModel
+                        _cover_model = _cover_session.query(GenerationTaskModel).filter(
+                            GenerationTaskModel.id == task_id
+                        ).first()
+                        if _cover_model:
+                            _cover_model.cover_url = cover_url
+                            _cover_session.commit()
+                            logger.info(
+                                "[task_id=%s] 封面URL已持久化: %s",
+                                task_id,
+                                cover_url[:80],
+                            )
+                    finally:
+                        _cover_session.close()
+                except Exception as cover_err:
+                    logger.warning(
+                        "[task_id=%s] 封面URL持久化失败（不影响主流程）: %s",
+                        task_id,
+                        cover_err,
+                    )
+
             _update_task_progress(task_id, 80, "渲染完成")
 
             # ── 4. 上传 OSS + 查重记录 ───────────────────────────────────────
@@ -1563,6 +1591,7 @@ def generate_video(self, task_id: str) -> dict:
                 editing_mode=editing_mode,
                 user_id=user_id,
                 video_name=task_info.get("video_title", ""),
+                thumbnail_url=cover_url,
             )
 
             if gen_task: