Package postgres provides a PostgreSQL-backed db.Store implementation for
Leaf, built on the jackc/pgx/v5 driver (PostgreSQL 18 compatible) through its
database/sql-compatible stdlib adapter.
Open connects to PostgreSQL using the given DSN (libpq URL or key/value DSN,
e.g. "postgres://user:pass@localhost:5432/game?sslmode=disable") and returns
a ready-to-use db.Store.