fix: add mailserver to front network for outbound internet access

mail-internal is internal:true (no internet). mailserver needs the
front network to resolve DNS and deliver emails to external MX servers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

roles/tools/templates/docker-compose.yml.j2

@@ -139,7 +139,8 @@ services:
     domainname: {{ domain_base }}
     restart: unless-stopped
     networks:
-      - mail-internal
+      - mail-internal   # Outline → mailserver (internal)
+      - front           # mailserver → internet (outbound delivery)
     environment:
       - SMTP_ONLY=1              # no IMAP/POP3, sending only
       - ENABLE_RSPAMD=0          # lightweight: skip spam filter