feat: async MX record validation (DNS lookup)

[polaz]

Summary

domain_check_mx() — verify domain has MX records via DNS. Separate feature dns with hickory-resolver.

Tasks

• Feature flag: dns (not default)
• EmailAddress::verify_mx(&self) -> Result<bool, DnsError> (async)
• Batch: verify_mx_batch(emails) -> Vec<Result<bool, DnsError>>
• Timeout config, custom resolver support
• Integration test with real DNS (not mocked)

Time estimate

4h