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/web-app-wordpress/vars/main.yml

@@ -4,7 +4,7 @@ database_type:              "mariadb"
 
 # WordPress
 WORDPRESS_URL:                "{{ domains | get_url(application_id, WEB_PROTOCOL) }}"
-WORDPRESS_MSMTP_SRC:          "{{ [ playbook_dir, 'roles/sys-svc-msmtp/templates/msmtprc.conf.j2' ] | path_join }}"
+WORDPRESS_MSMTP_SRC:          "{{ [ playbook_dir, 'roles/sys-svc-mail-msmtp/templates/msmtprc.conf.j2' ] | path_join }}"
 WORDPRESS_MSMTP_ABS:          "{{ [ docker_compose.directories.config, 'msmtprc.conf'] | path_join }}"
 WORDPRESS_MAX_UPLOAD_SIZE:    "{{ applications | get_app_conf(application_id, 'max_upload_size') }}"
 WORDPRESS_CUSTOM_IMAGE:       "wordpress_custom"