fix: default generate to PGlite (no Postgres required)

[theatlasteam]

Fixes #2
@algora-pbc /claim #2

Summary

• Default generate() and pgstrap generate to PGlite so generation works without a running PostgreSQL server.
• Add --no-pglite / pglite: false support for external PostgreSQL.
• Close the PGlite gateway and database and restore DATABASE_URL, POSTGRES_URI, and PG_URI in all cases.
• Scaffold db:generate for PGlite and document the default and opt-out behavior.
• Add a regression test covering default generation and environment restoration.

Test plan

• With no Postgres running: pgstrap generate (or bun run db:generate after init) succeeds
• With no Postgres running: bun test tests/generate.defaults.test.ts
• External PostgreSQL remains available with pgstrap generate --no-pglite