Scene detection lacks timeouts for ffprobe/ffmpeg

[codihuston]

Cause: subprocess.run calls have no timeout.
Analysis: Corrupt or malformed inputs can hang workers indefinitely and stall the pool.
Acceptance Criteria:

• ffprobe/ffmpeg calls time out and fail the task safely.
• Worker remains responsive under bad inputs.
  Solution Approach:
• Add timeouts and handle TimeoutExpired.
• Consider watchdog/heartbeat for long-running tasks.