feat(unified-render): Phase 2 - Feature Flag + RenderAdapter 适配层

- 新增 RENDER_ENGINE 配置项（legacy/unified），支持运行时切换渲染引擎
- 新建 RenderAdapter 适配层：EditPlan + EditPlanClips → ResolvedClip 转换，素材下载，进度回调
- compose_video 任务改造：根据 RENDER_ENGINE 配置选择旧引擎或新引擎
- 新引擎结果回写字段对齐（engine/width/height/file_size）
- 新增 16 个单元测试，validate_plan/render_plan/download_assets 全覆盖

apps/worker/video_processing/__init__.py

@@ -6,6 +6,7 @@
 from . import dedup_helpers, ffmpeg_utils, oss_helpers
 from .processor import VideoProcessor, VideoResult
 from .unified_render_service import RenderResult, UnifiedRenderService
+from .render_adapter import RenderAdapter, RenderAdapterResult
 
 __all__ = [
     "VideoProcessor",
@@ -15,4 +16,6 @@ __all__ = [
     "dedup_helpers",
     "UnifiedRenderService",
     "RenderResult",
+    "RenderAdapter",
+    "RenderAdapterResult",
 ]