Default generate to PGlite

[renaissancefieldlite]

Summary

• make pgstrap generate use the existing PGlite generation path by default
• scaffold db:generate as pgstrap generate --pglite so newly initialized projects do not need a background Postgres server
• keep external Postgres available through pgstrap generate --no-pglite
• restore DATABASE_URL and close the temporary PGlite gateway in a finally block
• update README and init script coverage

Verification

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

@algora-pbc /claim #2