ADD CONSTRAINT UNIQUE creates a supporting index under the hood; when
the index name already exists from initial-schema's inline UNIQUE,
Postgres raises SQLSTATE 42P07 (duplicate_table), not 42710
(duplicate_object) — so the EXCEPTION block didn't catch it and CI's
Migrations apply gate failed with `relation
"user_cards_user_id_card_id_is_foil_key" already exists`.
Swap to a pg_constraint pre-check: bulletproof against both SQLSTATEs
without overreaching to WHEN OTHERS.
Co-authored-by: Cursor <cursoragent@cursor.com>
a000c59ec4