go.mod: bump lib/pq to v1.12.3
v1.12.x adds sslmode=prefer/allow and protocol 3.2, and fixes a
broken-connection regression from v1.12.0 (v1.12.2). No code changes are
needed: *pq.Error and its Code still compare against the existing untyped
SQLSTATE constants.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>