monitoring: add nixos-exporter to all hosts

Add nixos-exporter prometheus exporter to track NixOS generation metrics
and flake revision status across all hosts.

Changes:
- Add nixos-exporter flake input
- Add commonModules list in flake.nix for modules shared by all hosts
- Enable nixos-exporter in system/monitoring/metrics.nix
- Configure Prometheus to scrape nixos-exporter on all hosts

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

system/monitoring/metrics.nix

@@ -18,4 +18,21 @@
       "--systemd.collector.enable-ip-accounting"
     ];
   };
+
+  services.prometheus.exporters.nixos = {
+    enable = true;
+    # Default port: 9971
+    flake = {
+      enable = true;
+      url = "git+https://git.t-juice.club/torjus/nixos-servers.git";
+    };
+  };
+
+  # Register nixos-exporter as a Prometheus scrape target
+  homelab.monitoring.scrapeTargets = [
+    {
+      job_name = "nixos-exporter";
+      port = 9971;
+    }
+  ];
 }