Add scripts/migrate_mysql_to_dm.py + .sh wrapper + example config: a single
data-only migration tool covering all three databases. Schema is owned by each
app's own DM path (create_all/init_dm.sql/openfga migrate); the script copies
rows with IDENTITY_INSERT, trigger/FK disabling, identity reseed, JSON->CLOB
conversion, --verify and --resume-from.
Remove src/backend/scripts/mysql_to_dm_migrate.py: bisheng-only and superseded
by the unified script (which also handles IDENTITY_INSERT and update_time
trigger disabling that the old script lacked).
GuoQing Zhang
3b1cb3307e