Additive/expand-contract: new append-only retained_identities table (the legal
dossier of every detached credential, keyed by account_id, TTL'd by detached_at)
plus nullable accounts columns last_login_at/last_login_ip (cold-load stamp) and
deleted_at/deleted_display_name (tombstone + retained real name). Regenerates the
go-jet models for accounts + retained_identities only.
Ilia Denisov