Archive a migration

Archives the migration, like Archive in the app — nothing is deleted. It drops out of GET /migrations (unless include_archived=true), stays readable and exportable, and every call that changes it (uploads, runs, answers, credentials, knowledge, the source connection, cancelling, PATCH) returns 409 with code: migration_archived. Restore it from the Archived page in Vern. Refused with 409 (code: imports_in_flight) while an import is in progress, including a preview waiting for approval — cancel the listed runs first. Archiving an archived migration is a no-op.