Default generate to PGlite

[mg272011]

@algora-pbc /claim #2

Summary

• makes pgstrap generate use the existing PGlite path by default so generated projects can run db:generate without a local Postgres server
• keeps external Postgres generation available through --no-pglite
• restores DATABASE_URL and closes the temporary PGlite gateway server in a finally block
• updates README guidance and regression coverage for the default PGlite path

Verification

• bun test
• bun run format:check
• bun run build
• git diff --check