Remove synthetic setup bootstrap user

[mrubens]

Summary

• allow bootstrap/system environment-variable writes to use nullable audit attribution
• remove creation and special handling of the synthetic setup-bootstrap-user record
• reject queued onboarding launches that no longer have a selecting user
• add the generated migration that makes environment-variable audit user IDs nullable

Validation

• pnpm --filter @roomote/web check-types
• pnpm --filter @roomote/db check-types
• pnpm --filter @roomote/web lint
• pnpm --filter @roomote/db lint
• pre-push lint:fast, check-types:fast, and knip

Focused Vitest runs were attempted but the shared test Postgres returned global/pg_filenode.map I/O errors. Shared infrastructure was not restarted.