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-svc-mail-smtp/tasks/01_core.yml

@@ -0,0 +1,26 @@
+- include_tasks: utils/once/flag.yml
+
+- name: "Ensure msmtp-mta is absent (conflicts with postfix smtp-forwarder)"
+  community.general.pacman:
+    name:
+      - msmtp-mta
+    state: absent
+
+- name: "Install local SMTP relay (Postfix)"
+  community.general.pacman:
+    name:
+      - postfix
+    state: present
+
+- name: "Configure Postfix as localhost-only relay"
+  ansible.builtin.template:
+    src: "postfix-main.cf.j2"
+    dest: "/etc/postfix/main.cf"
+    mode: "0644"
+
+- name: "Ensure postfix is enabled and running"
+  ansible.builtin.systemd:
+    name: postfix
+    enabled: true
+    state: started
+  when: not( IS_CONTAINER | bool )