Fix NVENC transcode: use system ffmpeg instead of bundled one

Bundled ffmpeg at backend/ffmpeg/linux_x64/ffmpeg has no NVENC support.
System /usr/bin/ffmpeg does. Switch _ensure_h264 to use shutil.which('ffmpeg')
which resolves to /usr/bin/ffmpeg. Also log the actual NVENC error message
so future failures are visible in the task logs.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

web_app.py

@@ -406,23 +406,21 @@ def _ensure_h264(task_id: str, video_path: str) -> str:
 
     push_log(task_id, "⚠ 视频格式（可能是 AV1/HEVC）OpenCV 无法解码，自动转换为 H.264…")
     try:
-        from backend.tools.ffmpeg_cli import FFmpegCLI
-        import subprocess
+        import subprocess, shutil
+        # 优先系统 ffmpeg（有 NVENC），项目自带的没有编译 NVENC 支持
+        sys_ffmpeg = shutil.which('ffmpeg') or '/usr/bin/ffmpeg'
+        push_log(task_id, f"FFmpeg 转码中（H.264 NVENC）…")
         cmd = [
-            FFmpegCLI.instance().ffmpeg_path,
-            '-y', '-i', video_path,
+            sys_ffmpeg, '-y', '-i', video_path,
             '-c:v', 'h264_nvenc', '-cq', '18', '-preset', 'p4',
-            '-c:a', 'copy',
-            '-loglevel', 'error',
-            converted,
+            '-c:a', 'copy', '-loglevel', 'error', converted,
         ]
-        push_log(task_id, "FFmpeg 转码中（H.264 NVENC）…")
         result = subprocess.run(cmd, capture_output=True, text=True, timeout=3600)
         if result.returncode != 0 or not Path(converted).exists():
-            push_log(task_id, "NVENC 转码失败，回退 libx264…")
+            err = (result.stderr or '').strip()[:200]
+            push_log(task_id, f"NVENC 失败（{err or 'unknown'}），回退 libx264…")
             cmd_x264 = [
-                FFmpegCLI.instance().ffmpeg_path,
-                '-y', '-i', video_path,
+                sys_ffmpeg, '-y', '-i', video_path,
                 '-c:v', 'libx264', '-crf', '18', '-preset', 'fast',
                 '-c:a', 'copy', '-loglevel', 'error', converted,
             ]