docs: comprehensive changelog update for all changes since v3.1.3

Add missing video pipeline feature (the main 15K+ line addition),
15 video bug fixes, and restructure [Unreleased] section with
proper hierarchy: Video Pipeline Core → Video --setup → Word support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yusufkaraaslan