-- Apply after 20260909_watch_sessions.sql.
-- Preflight: this intentionally fails if historical data has more than one
-- active session per student. Reconcile those rows to ended/invalidated first.
ALTER TABLE watch_sessions
  ADD COLUMN active_student_id BIGINT UNSIGNED
    GENERATED ALWAYS AS (CASE WHEN status = 'active' THEN student_id ELSE NULL END) STORED,
  ADD UNIQUE KEY uq_watch_one_active_per_student (active_student_id);

-- One decision per event, including repeated positions that are legitimately
-- excluded. Position pairs alone are not an event identity.
ALTER TABLE eligible_watch_intervals
  DROP INDEX uq_eligible_interval,
  ADD COLUMN event_sequence_no INT UNSIGNED NULL AFTER watch_session_id,
  ADD UNIQUE KEY uq_watch_interval_event (watch_session_id, event_sequence_no, policy_version);

-- Rollback: stop writers that rely on the invariant first. Dropping the key
-- weakens protection and is not a routine production rollback.