Feature: configurable ffmpeg timeout for Manim/compose (avoid false ERROR on long scenes)

[jmjava]

Problem

During docgen compose, ffmpeg can log ERROR: ffmpeg timed out on long Manim segments while still producing output (user saw this on a ~260s+ architecture video).

Proposal

• Expose timeout (seconds) in docgen.yaml (e.g. under compose: or ffmpeg:) and/or CLI --ffmpeg-timeout.
• Default high enough for 1080p30 multi-minute scenes or document recommended value.

Acceptance criteria

• Timeout configurable
• Log level distinguishes hard failure vs retry/warning if applicable