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/resolv.conf.j2

@@ -0,0 +1,11 @@
+# Managed by KNELIAC — do not edit; changes will be overwritten.
+# Redundant recursive DNS via pfv-netinfra-01/02 (Technitium + Pi-hole).
+# IPs are used (required: nameserver directives must be addresses, and the
+# knel.net name for these hosts resolves to a Tailscale CGNAT address rather
+# than the LAN address). If the primary is unreachable, glibc's resolver
+# automatically falls through to the secondary.
+domain {{ dns_search_domain }}
+search {{ dns_search_domain }}
+{% for ns in dns_servers %}
+nameserver {{ ns }}
+{% endfor %}