Fix can_use_transactional_email field name mismatch and update SRS forwarding tests

[Copilot]

ForwarderServer._process_message referenced domain.can_use_transactional_relay — a field that doesn't exist on the Domain model (the actual field is can_use_transactional_email), silently disabling transactional relay routing. The SRS forwarding tests also broke when _rewrite_mailfrom was removed in favour of inline self.srs.forward() calls.

maildaemons/forwarder/server.py

• Fix domain.can_use_transactional_relay → domain.can_use_transactional_email in both the runtime routing condition and the --transactional-relay-ip CLI help text.
• transactional_relay as a parameter/argument name is correct and unchanged — it names the SMTP relay, not the model field.

mailauth/tests/test_mailforwarder_srs.py

• Replace test_rewrite_mailfrom_uses_static_sender_without_srs / test_rewrite_mailfrom_uses_srs_when_available (tested the removed _rewrite_mailfrom method) with _process_message-level equivalents:
  • test_process_message_uses_list_mailfrom_when_set — forward_to.new_mailfrom is used directly as envelope sender when set.
  • test_process_message_uses_srs_when_list_mailfrom_not_set — SRS rewrites the sender when new_mailfrom is empty.
• Fix test_process_message_uses_rewritten_mailfrom_for_forwarding → renamed test_process_message_list_mailfrom_overrides_srs_when_set: add missing domain=SimpleNamespace(name=...) on the alias (the current code calls alias.domain.name), remove mock of the deleted _rewrite_mailfrom, clarify that a non-empty new_mailfrom wins even when SRS is configured.

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