The Prisma unique-constraint (P2002) narrowing predicate had been
hand-copied into five call sites. Fold them into a single isP2002 in
src/lib/prisma-errors.ts and import it everywhere (the two step-drain
sites keep their local isUniqueConstraintViolation name via an import
alias). Behaviour-identical; one place to chase a future P2002-semantics
change instead of five.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>