CREATE DATABASE doesn't work inside a DO $$ ... $$ block (transactional
restriction). And psql `:'var'` substitutions resolve client-side, so
they can't reach inside a DO block either.
Replace both DO blocks with psql-native idioms:
- `\gexec` for the conditional CREATE DATABASE
- `\if` + `\gset` for the create-or-rotate-password branch on the
brain_app role
Verified end-to-end on koala postgres18: brain DB created, vector
0.8.1 extension installed, brain_app role login works.
7a13c75655