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/api/app/api/routes/generation_cover.py

@@ -185,27 +185,39 @@ def generate_cover(
             detail=f"获取预览视频URL失败: {e}",
         ) from e
 
-    # 优先使用渲染时预抽的封面候选帧（跳过 MediaKit，秒级返回）
-    cover_candidates = (plan.config or {}).get("cover_candidates", [])
-    if cover_candidates and body.cover_type in ("ai_frame", "ai_regenerate"):
-        logger.info(
-            "[封面生成] 使用预存封面候选帧: plan_id=%s count=%d",
-            plan_id,
-            len(cover_candidates),
-        )
-        first_frame = cover_candidates[0]
-        cover_data = {
-            "type": "ai_frame",
-            "image_url": first_frame.get("image_url", ""),
-            "frame_time": first_frame.get("frame_time", 0.0),
-            "confidence": 0.9,
-        }
-        if cover_data["image_url"]:
-            current_config = dict(plan.config) if plan.config else {}
-            current_config["cover"] = cover_data
-            normalized = normalize_plan_config(current_config)
-            plan_svc.update_plan_config(plan_id, {"cover": normalized["cover"]})
-            return GenerateCoverResponse(plan_id=plan_id, cover=cover_data)
+    # 统一封面管道：优先从 GenerationTask.cover_url 读取渲染后视频抽帧的封面
+    if body.cover_type in ("ai_frame", "ai_regenerate"):
+        # 尝试从 GenerationTask 读取已持久化的封面 URL
+        generation_task_id = (plan.config or {}).get("generation_task_id", "")
+        if generation_task_id:
+            try:
+                gen_task_repo = SQLAlchemyGenerationTaskRepository(db)
+                task = gen_task_repo.get(generation_task_id)
+                if task and getattr(task, "cover_url", ""):
+                    cover_data = {
+                        "type": "ai_frame",
+                        "image_url": task.cover_url,
+                        "frame_time": 0.0,
+                        "confidence": 0.95,
+                    }
+                    logger.info(
+                        "[封面生成] 使用统一管道封面: plan_id=%s task_id=%s url=%s",
+                        plan_id,
+                        generation_task_id,
+                        task.cover_url[:80],
+                    )
+                    current_config = dict(plan.config) if plan.config else {}
+                    current_config["cover"] = cover_data
+                    normalized = normalize_plan_config(current_config)
+                    plan_svc.update_plan_config(plan_id, {"cover": normalized["cover"]})
+                    return GenerateCoverResponse(plan_id=plan_id, cover=cover_data)
+            except Exception:
+                logger.warning(
+                    "[封面生成] 读取 GenerationTask.cover_url 失败: plan_id=%s task_id=%s",
+                    plan_id,
+                    generation_task_id,
+                    exc_info=True,
+                )
 
     from packages.shared.ai_service import run_generate_cover