Interface PgQueryResult<R>

Minimal structural subset of the pg (node-postgres) client surface that the PostgreSQL backend relies on.

We deliberately depend on this tiny local interface instead of @types/pg so that:

• Redis-only users never need pg (or its types) installed — the driver is an optional dependency that is lazy-required by the factory.
• The migrator and SQL helpers stay trivially unit-testable with a fake queryable, and remain easy to port conceptually to other runtimes.

Any real pg.Client / pg.Pool / pg.PoolClient is structurally assignable to PgQueryable.