feat: multi-domain support (apimurah.my.id, payin.my.id), expanded random words, fix DB uniqueness check

- Add apimurah.my.id & payin.my.id as additional mail domains with routes
- Parse MAIL_DOMAIN as comma-separated list with getDomains() helper
- API /config now returns mailDomains array alongside mailDomain for backward compat
- POST /api/inboxes validates domain against allowed list, rejects unknown domains
- Web UI gets domain selector dropdown populated from config
- Expand random address word lists from 30 to ~100 Indonesian words each
- Fix generateUniqueAddress to check actual DB (inboxExists) instead of session Set
- Make generateUniqueAddress async to support Promise-based DB checks

Co-Authored-By: Claude <noreply@anthropic.com>