video: fix flush of closed file error

Use wait() instead of communicate() after manually closing stdin.
communicate() tries to flush stdin which fails if already closed.

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

backend/video_generator/create_video.py

@@ -193,7 +193,8 @@ def generate_and_stream_frames(base_img, pet_img, vinyl_img, needle_img, audio_d
                 logger.info(f"Frame progress: {frame_num + 1}/{frames_per_rotation}")
 
         ffmpeg_process.stdin.close()
-        stdout, stderr = ffmpeg_process.communicate()
+        ffmpeg_process.wait()
+        stderr = ffmpeg_process.stderr.read()
 
         if ffmpeg_process.returncode != 0:
             logger.error(f"FFmpeg error: {stderr.decode()}")