modules: add homelab.host for host metadata

Add a shared `homelab.host` module that provides host metadata for
multiple consumers:
- tier: deployment tier (test/prod) for future homelab-deploy service
- priority: alerting priority (high/low) for Prometheus label filtering
- role: primary role of the host (dns, database, monitoring, etc.)
- labels: free-form labels for additional metadata

Host configurations updated with appropriate values:
- ns1, ns2: role=dns with dns_role labels
- nix-cache01: priority=low, role=build-host
- vault01: role=vault
- jump: role=bastion
- template, template2, testvm01, vaulttest01: tier=test, priority=low

The module is now imported via commonModules in flake.nix, making it
available to all hosts including minimal configurations like template2.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/homelab/default.nix

@@ -2,6 +2,7 @@
 {
   imports = [
     ./dns.nix
+    ./host.nix
     ./monitoring.nix
   ];
 }