Migration 28 (add_snapshot_hot_path_indexes) referenced task_comments,
documents, meetings, sprints, agent_messages, activity — all tables
created by ensureOptionalColumns() which runs AFTER runMigrations() on
a fresh DB. Converted from sql-string migration to fn-based so each
index is gated by tableExists(); safe on both fresh and migrated DBs.
Also updated tests/unit/selective-ipc.test.ts exhaustive switch to
include the 7 new DesktopEvent variants added in ef90a58.