-- Planning Schedule — add is_frozen column
-- Marks past rows as frozen so Calc never deletes them.
-- is_frozen = 1: row is historical — Calc DELETE skips it
-- is_frozen = 0: row is live — Calc may replace it
-- Set by PlanningCron::freezePastRows() (runs nightly).
-- Created: 2026-04-11
-- NOTE: applied via planning_migrate.php (planningEnsureIsFrozenColumn)
