Add /api/v2 auth/status/migrate endpoints for the three OAuth-based
migrators. One generic helper registers all three ops per migrator
behind its static config gate, so there's no copy-pasted block per
migrator.
The migrate kick-off orchestration (already-running guard + event
dispatch) is extracted into migrationHandler.StartMigration so v1 and
v2 share it; v1's wire output is unchanged. The guard now surfaces as a
typed migration.ErrMigrationAlreadyRunning (412) so v2 can translate it
through the standard error bridge.
kolaente