Default scaffolded db:generate to PGlite

[JYZ-LESLIE]

Summary

• change pgstrap init scaffold so db:generate runs pgstrap generate --pglite
• update init test expectation
• update README script description to match new default behavior

This makes bun run db:generate work without requiring a local Postgres process.

Validation

• Could not run tests locally in this environment because bun is not installed.

@algora-pbc /claim #2