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>

hosts/testvm01/configuration.nix

@@ -16,6 +16,11 @@
   # Test VM - exclude from DNS zone generation
   homelab.dns.enable = false;
 
+  homelab.host = {
+    tier = "test";
+    priority = "low";
+  };
+
   nixpkgs.config.allowUnfree = true;
   boot.loader.grub.enable = true;
   boot.loader.grub.device = "/dev/vda";