feat(worker): ffmpeg渲染失败时捕获stderr写入error_detail，增强排障能力

- RenderAdapterResult新增error_detail字段存完整stderr
- render_adapter单独捕获CalledProcessError，把stderr写入error_message和error_detail
- edit_plan_generation失败时把error_detail拼进gen_task.error_message
- 新增test_ffmpeg_error_captures_stderr测试
- 35个render_adapter测试全绿

apps/worker/video_processing/render_adapter.py

@@ -16,6 +16,7 @@
 from __future__ import annotations
 
 import logging
+import subprocess
 import tempfile
 from dataclasses import dataclass
 from pathlib import Path
@@ -70,6 +71,7 @@ class RenderAdapterResult:
     rendered_clip_ids: list[str] = None  # 成功渲染的 clip id 列表
     failed_clip_ids: list[str] = None  # 失败的 clip id 列表
     error_message: str = ""
+    error_detail: str = ""  # 详细错误信息（如 ffmpeg stderr），用于排查
 
     def __post_init__(self):
         if self.rendered_clip_ids is None:
@@ -249,6 +251,20 @@ class RenderAdapter:
                 failed_clip_ids=failed_clip_ids,
             )
 
+        except subprocess.CalledProcessError as exc:
+            stderr_text = (exc.stderr or "").strip()
+            logger.error(
+                "[render-adapter] ffmpeg渲染失败: plan_id=%s job_id=%s exit_code=%d\nstderr:\n%s",
+                plan_id,
+                job_id,
+                exc.returncode,
+                stderr_text[:3000],
+            )
+            return RenderAdapterResult(
+                success=False,
+                error_message=f"FFmpeg渲染失败(exit={exc.returncode}): {stderr_text[:200]}",
+                error_detail=stderr_text[:3000],
+            )
         except Exception as exc:
             logger.exception(
                 "[render-adapter] render failed: plan_id=%s job_id=%s engine=unified error=%s",