After reconcileRenumberedMigrations() rewrites a legacy row (e.g. 028→029),
verify the old version slot is clear. A residual row at the old version
would cause getAppliedVersions() to skip the new migration file at that
version number (028_create_files_and_batches.sql).
Also adds 'batches' table to PHYSICAL_SCHEMA_SENTINELS for physical
schema recovery on upgrade paths.