A taken address on PostgreSQL fell through to a generic error, because
the store looked for SQLite's "UNIQUE". It checks code 23505 too. The
done page tells someone whose address had an account to sign in or ask
for a reset, without saying which case it was; other failures give a
logged reference.
Refs #2 (item 6).
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
