Use dedicated Rowset domain for transactional email

[gregagi]

Summary

• Use dedicated mg.rowset.app for all Django/allauth transactional emails, with Rowset <rasul@rowset.app> sender and environment overrides.
• Document both-service configuration, scoped credential storage, verification, and rollback; leave signup rules and newsletter integrations unchanged.
• Cover sender defaults and signup/confirmation/password-reset/existing-account email adapter wiring.

Verification

• Regression reproduced against the previous LVTD sender defaults, then five focused tests passed (external MJML renderer mocked for unit tests).
• Ruff and diff checks passed. Full backend/frontend/CLI checks run in CI.
• Mailgun domain active, all five required DNS records valid, tracking disabled; operator-inbox domain smoke email passed SPF/DKIM/DMARC.
• Production runtime/account-email delivery checks follow deployment. No customer test email, Listmonk, or newsletter setup.