feat(system-config): add system service configuration role

Port global-systemServiceConfigurationFiles() and
global-postPackageConfiguration().

Manages:
- Global zsh config (/etc/zshrc)
- Mail aliases and postfix relay (local-only, smarthost to Cloudron)
- Static DNS resolv.conf (replaces systemd-resolved symlinks)
- rsyslog forwarding to LibreNMS (skipped on LibreNMS server itself)
- DHCP client config (skipped on DHCP servers pfv-netinfra-01/02)
- SNMP daemon config selected by host class (Pi/physical/Proxmox/VM)
- NTP client config (skipped on NTP server hosts)
- LLDP network discovery
- Cockpit disallowed-users
- Default user shells (zsh, conditional on user existence)
- Process accounting (accton)
- CPU governor on physical/Proxmox hosts (performance)
- tuned profile on VM guests (virtual-guest)

Infrastructure configs use Jinja2 templates parameterized by variables
in group_vars/all.yml.

🤖 Generated with [Crush](https://github.com/charmassociates/crush)

Assisted-by: GLM-5 via Crush <crush@charm.land>

roles/system_config/templates/dhclient.conf.j2

@@ -0,0 +1,14 @@
+option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;
+
+send host-name = gethostname();
+request subnet-mask, broadcast-address, time-offset, routers,
+        domain-name, host-name,
+        domain-name-servers, domain-search, ntp-servers,
+        rfc3442-classless-static-routes;
+
+# Pin DNS and NTP to the redundant pfv-netinfra-01/02 pair regardless of what
+# the DHCP server advertises, so every host on this build uses the same
+# authoritative recursive resolvers and time sources.
+supersede domain-name-servers {{ dns_servers | join(', ') }};
+supersede domain-search "{{ dns_search_domain }}";
+supersede ntp-servers {{ ntp_servers | join(', ') }};