feat(biz): 更新FFmpeg任务启动功能以支持环境变量配置最大工作线程数

- 修改start_ffmpeg_task函数参数max_workers默认值为None
- 添加环境变量FFMPEG_MAX_WORKERS读取逻辑
- 当max_workers为None时从环境变量获取默认值，否则使用传入值
- 保持原有tracer和任务分析功能不变

biz/ffmpeg.py

@@ -129,7 +129,9 @@ def check_placeholder_exist_with_count(placeholder_id, task_params, required_cou
     return False
 
 
-def start_ffmpeg_task(ffmpeg_task, max_workers=4):
+def start_ffmpeg_task(ffmpeg_task, max_workers=None):
+    if max_workers is None:
+        max_workers = int(os.environ.get("FFMPEG_MAX_WORKERS", 4))
     tracer = get_tracer(__name__)
     with tracer.start_as_current_span("start_ffmpeg_task") as span:
         sub_tasks = list(ffmpeg_task.analyze_input_render_tasks())