Choosing a calendar other than the stored one now clears every plan day's calendar_event_id and edits each plan that had one (revision raised, synced -> changed) in the same transaction as the choice, so a stored event id is always on the chosen calendar and the next push writes the whole week there. PushEvents now runs the push's record under the same account-lock hold as its event writes, so the chosen calendar cannot change between the writes and the sync bookkeeping; a push that read a plan before the choice is refused by its revision check (ErrPlanChanged). Fixes review finding F-01 on task 5885d2b1-cf06-44ff-abf3-242d7763c3c9. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>