Harden published curriculum and student flows

backend/migrations/20260923_watch_single_active_session.sql

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+-- 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.