CREATE ROLE ... PASSWORD :'teecup_app_password' silently failed with
"syntax error at or near ':'" because the whole DO $$ ... $$ body is a
dollar-quoted string and psql skips variable substitution inside it.
Replaced the idempotency check with \gset + \if/\else so the PASSWORD
line is plain top-level SQL, where interpolation does work. Verified
against a scratch database (see test_isolation.sql run).
