- owner_id (FK users, ON DELETE RESTRICT) + index on (owner_id, created_at DESC)
- client_id (nullable FK clients, ON DELETE RESTRICT) for client back-references
- issuer_snapshot jsonb + snapshot_version: invoices freeze the issuer's
legal-entity fields at issue time so historical PDFs render unchanged when
the user later edits their profile
- voided_at + voided_reason: soft-void instead of delete, since invoices
are retained indefinitely
- swap unique(number) for unique(owner_id, number): numbers are now per-user,
not global
DROP CONSTRAINT uses IF EXISTS so the migration is resilient to the
auto-generated constraint name differing across Postgres versions.
