修复LAMA模式100%卡死：帧区间扩展超出视频总帧数导致FramePrefetcher死锁

- 限制字幕区间end不超过frame_count，防止内循环消费哨兵后外层永久阻塞
- LAMA批量推理改为mini-batch(4帧)，避免GPU OOM
- 各inpaint模型空inpaint_area时返回原始帧
- FFmpeg子进程添加600s超时保护

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tools/video_io.py

@@ -97,4 +97,8 @@ class FFmpegVideoWriter:
             self._process.stdin.close()
         except BrokenPipeError:
             pass
-        self._process.wait()
+        try:
+            self._process.wait(timeout=600)
+        except subprocess.TimeoutExpired:
+            self._process.terminate()
+            self._process.wait(timeout=5)