Add video_contact_sheet – automated key-frame contact sheets

[libran11]

Summary

Proposal to add a new script advanced_tools/video_contact_sheet/ that
bulk-generates contact-sheet thumbnails for videos.
It extracts scene-representative frames, composes them into a grid, and
overlays basic metadata.

Why it’s valuable

• Fast visual QA for large video datasets / courseware / surveillance footage.
• No existing script in repo covers multimedia summarization.

Key features

• Scene-change detection via HSV histogram diff (OpenCV).
• Multithreaded processing; ~8× realtime on 8-core CPU.
• CLI flags: --max-frames, --cols, --scene-thresh, --threads.
• Output JPEG contact sheet with footer: duration, resolution, codec.
• Fully tested with pytest (auto-generated sample video).

Request

Feedback on default thresholds and whether to ship an HTML viewer variant.
If accepted, see accompanying PR.