pg-store: coerce postmodern :NULL to NILVerified

postmodern returns SQL NULL as the keyword :NULL (truthy). store-find-auth-code
read a fresh code's NULL consumed_at as :NULL, so redeem-code falsely reported
"authorization code replay" on the very first use (PG backend only; SQLite/in-
memory return NIL). Coerce rows through %nn in the user/client/auth-code readers.
Regression check added to pg-smoke.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>