Add a skip_on_away flag to quest_definitions (migration 8, additive
column defaulting to 1 so existing rows keep today's behaviour). The
admin create/edit routes accept it as an optional boolean (create
defaults true, an omitted edit field keeps the stored value) and the
definition payload returns it. Materialisation generates instances on
absent dates when the flag is false.
Task: ded75ab8-0219-4d52-aefa-82c09e0b5e6b
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>