Use PGlite for generated db:generate script

[SimoneMariaRomeo]

Summary

• make newly initialized db:generate scripts run pgstrap generate --pglite, so bun run db:generate no longer needs a local Postgres service
• document the generated script behavior while keeping pgstrap generate as the explicit configured-Postgres path
• wrap the temporary PGlite gateway in cleanup so DATABASE_URL is restored and the server is closed after generation
• add coverage for the generated script and DATABASE_URL restoration

Fixes #2

Verification

• bunx biome format README.md src/generate.ts src/init.ts tests/generate.pglite.test.ts tests/init.test.ts
• bun test
• bun run build

Note: full bun run format:check rewrites untouched files on Windows because the checkout has CRLF line endings, so I limited formatting to the changed files.

@algora-pbc /claim #2