Make db:generate run through PGlite by default

[MANFIT7]

@algora-pbc /claim #2

Closes #2

Summary

• scaffolds db:generate as pgstrap generate --pglite so new projects do not need a running Postgres server for generated types and structure dumps
• makes direct pgstrap generate use PGlite by default, with --no-pglite as the external-Postgres opt-out
• restores DATABASE_URL and closes the temporary PGlite gateway server in finally
• adds regression coverage for default PGlite generation and the generated init script

Verification

• bun test
• bun run format:check
• PATH="/Applications/Codex.app/Contents/Resources:$PATH" bun run build
• git diff --check