Aggregate sessions grouped by session number (MED-702)

Returns the cohort's sessions re-shaped for the "View by Session" toggle. For modality=group_coaching, yields one entry per session number with the session-number-level topic, aggregated attendance/engagement, and the list of groups whose session with that number feeds into the roll-up. For modality=one_on_one, yields one entry per session number with the propagated session type, scheduled/completed counts, and the participant row list. When coachUserId is supplied, the result is scoped to that coach's assigned groups (group coaching) or assigned participants (1:1).