feat(db): add node-postgres driver support

[adamkasper]

Adds explicit node-postgres support for NuxtHub DB.

• Drizzle documents PlanetScale Postgres through drizzle-orm/node-postgres.
• NuxtHub users can now opt into that driver with driver: 'node-postgres'.
• Keeps existing PostgreSQL auto-detection unchanged: default remains postgres-js, with PGlite fallback only for the default path.

Also updates docs with per-driver PostgreSQL install guidance and adds focused tests for config resolution and client creation.

[vercel]

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