Bumps _SOULSYNC_BASE_VERSION 2.6.8 → 2.6.9, the docker-publish workflow's
default version tag, and adds the 2.6.9 What's New entry (15 items, security
fixes first: #832 launch-PIN enforcement and the settings-secret leak, then
#833/#831/#830/#829/#828/#827/#825/#824/#823/#740, Spotify (no auth), multi-
artist tags, decimal-volume dedup).