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>

flake.lock

@@ -42,6 +42,26 @@
         "url": "https://git.t-juice.club/torjus/labmon"
       }
     },
+    "nixos-exporter": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs-unstable"
+        ]
+      },
+      "locked": {
+        "lastModified": 1770417341,
+        "narHash": "sha256-Js2EJZlpol6eSWVGyYy6SAhIISKHfURnD6S+NBaLfHU=",
+        "ref": "refs/heads/master",
+        "rev": "e381038537bbee314fb0326ac7d17ec4f651fe1e",
+        "revCount": 7,
+        "type": "git",
+        "url": "https://git.t-juice.club/torjus/nixos-exporter"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://git.t-juice.club/torjus/nixos-exporter"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1770136044,
@@ -78,6 +98,7 @@
       "inputs": {
         "alerttonotify": "alerttonotify",
         "labmon": "labmon",
+        "nixos-exporter": "nixos-exporter",
         "nixpkgs": "nixpkgs",
         "nixpkgs-unstable": "nixpkgs-unstable",
         "sops-nix": "sops-nix"