chore: bump version to 0.15.0

fix: technical footer now appears in streaming responses

fix: /model selector opens instantly without waiting for lock

fix: respect cron enabled flag on reschedule and execution

fix: show busy info only after actual model switch, not on selector open

fix: add fallback to main user when heartbeat delivery fails

fix: pass send opts to Matrix heartbeat delivery

fix: deliver heartbeat messages to correct forum topic

docs: update all RULES files with new config sections

feat: independent heartbeat loops per group target

feat: per-target enabled flag and heartbeat hot-reload start/stop

fix: translate restart message and add language hot-reload

feat: seed heartbeat group_targets with example entry on first load

feat: add NL, ES, FR, PT, RU translations

docs: add language, image, scene, and heartbeat group_targets to example config