Make mail stack optional for Infinito.Nexus deployments without Mailu (e.g. Raspberry Pi / robots)

Refactored mail-related roles to support running Infinito.Nexus on nodes without a dedicated mail server:
- Introduced sys-svc-mail as central mail orchestration role.
- Split msmtp handling into sys-svc-mail-msmtp.
- Added sys-svc-mail-smtp to provide a localhost-only Postfix relay when Mailu is not present.
- Updated alert/health roles to use the new mail orchestration.
- Avoid installing postfix inside containers via IS_CONTAINER guard.
- Adjusted WordPress role to use the new msmtp template path.

This allows lightweight deployments (e.g. Raspberry Pi, robots, edge nodes) to send mail via localhost without requiring a full Mailu stack.

ChatGPT discussion: https://chatgpt.com/share/6931edf1-cb98-800f-9e3c-a62d69ccb223

roles/sys-ctl-alm-compose/tasks/01_core.yml

@@ -36,4 +36,6 @@
     ansible.builtin.systemd:
       name: "{{ system_service_id | get_service_name(SOFTWARE_NAME, False) ~ escaped_name.stdout }}.service"
       state: started
-  when: MODE_ASSERT | bool
+  when:
+    - MODE_ASSERT | bool
+    - not( IS_CONTAINER | bool )