fix(worker): 改进ffmpeg错误信息可见性，stderr尾部优先+error_detail不丢弃

1. error_message显示stderr尾部500字符（而非头部ffmpeg banner）
2. error_detail保留stderr尾部5000字符
3. generation任务RuntimeError保留error_detail
4. 缩略图ffmpeg错误同理

apps/worker/video_processing/render_adapter.py

@@ -221,17 +221,18 @@ class RenderAdapter:
 
         except subprocess.CalledProcessError as exc:
             stderr_text = (exc.stderr or "").strip()
+            stderr_tail = stderr_text[-5000:] if len(stderr_text) > 5000 else stderr_text
             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[-2000:] if len(stderr_text) > 2000 else stderr_text,
+                stderr_tail,
             )
             return RenderAdapterResult(
                 success=False,
-                error_message=f"FFmpeg渲染失败(exit={exc.returncode}): {stderr_text[:200]}",
-                error_detail=stderr_text[-2000:] if len(stderr_text) > 2000 else stderr_text,
+                error_message=f"FFmpeg渲染失败(exit={exc.returncode}): {stderr_text[-500:]}",
+                error_detail=stderr_tail,
             )
         except Exception as exc:
             logger.exception(
@@ -721,16 +722,17 @@ class RenderAdapter:
 
         except subprocess.CalledProcessError as exc:
             stderr_text = (exc.stderr or "").strip()
+            stderr_tail = stderr_text[-5000:] if len(stderr_text) > 5000 else stderr_text
             logger.error(
                 "[render-adapter] 内存模式渲染失败: plan_id=%s exit_code=%d\nstderr:\n%s",
                 actual_plan_id,
                 exc.returncode,
-                stderr_text[-2000:] if len(stderr_text) > 2000 else stderr_text,
+                stderr_tail,
             )
             return RenderAdapterResult(
                 success=False,
-                error_message=f"FFmpeg渲染失败(exit={exc.returncode}): {stderr_text[:200]}",
-                error_detail=stderr_text[-2000:] if len(stderr_text) > 2000 else stderr_text,
+                error_message=f"FFmpeg渲染失败(exit={exc.returncode}): {stderr_text[-500:]}",
+                error_detail=stderr_tail,
             )
         except Exception as exc:
             logger.exception(