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/templates/postfix-main.cf.j2

@@ -0,0 +1,8 @@
+# roles/sys-svc-msmtp/templates/postfix-main.cf.j2
+myhostname = {{ inventory_hostname }}
+inet_interfaces = loopback-only
+mydestination =
+relayhost =
+mynetworks = 127.0.0.0/8
+local_transport = error: local delivery disabled
+default_transport = smtp