fix(mail): allow cascading additional domains

[Rajioba1]

Summary

• scope postmaster hard-delete protection to the primary install domain
• allow automatically-created postmaster mailboxes on additional domains to be removed during cascade deletion
• fail closed when the install domain cannot be read
• add regression coverage for additional, primary, and unavailable-state cases

Verification

• bunx vitest run test/modules/mail/mailboxes.service.test.ts test/modules/mail/dns-scan.service.test.ts test/modules/mail/psql-injection.test.ts (11 tests passed)
• bun run --cwd apps/api lint
• Prettier check on both touched files
• git diff --check

Formatting note

mailboxes.service.ts was not Prettier-clean on main. Per CONTRIBUTING.md, the formatting-only changes are isolated in the first commit; the functional change and tests are in the second commit.

Fixes #76