monitoring: add zpool and boot partition usage metrics

Add textfile collector for ZFS pool utilization (tank, hdds) and
boot drive partitions (/boot, /persistent, /nix). Runs every 60s.
Add two Grafana dashboard panels: ZFS Pool Utilization and Boot
Drive Partitions as Row 5.

services/monitoring.nix

@@ -79,6 +79,17 @@ let
     '';
   };
 
+  diskUsageCollector = pkgs.writeShellApplication {
+    name = "disk-usage-collector";
+    runtimeInputs = with pkgs; [
+      coreutils
+      gawk
+      config.boot.zfs.package
+      util-linux # for mountpoint
+    ];
+    text = builtins.readFile ./disk-usage-collector.sh;
+  };
+
   dashboard = {
     editable = true;
     graphTooltip = 1;
@@ -669,6 +680,94 @@ let
           overrides = [ ];
         };
       }
+
+      # -- Row 5: Storage --
+      {
+        id = 12;
+        type = "timeseries";
+        title = "ZFS Pool Utilization";
+        gridPos = {
+          h = 8;
+          w = 12;
+          x = 0;
+          y = 32;
+        };
+        datasource = promDs;
+        targets = [
+          {
+            datasource = promDs;
+            expr = "zpool_used_bytes{pool=\"tank\"} / zpool_size_bytes{pool=\"tank\"} * 100";
+            legendFormat = "tank";
+            refId = "A";
+          }
+          {
+            datasource = promDs;
+            expr = "zpool_used_bytes{pool=\"hdds\"} / zpool_size_bytes{pool=\"hdds\"} * 100";
+            legendFormat = "hdds";
+            refId = "B";
+          }
+        ];
+        fieldConfig = {
+          defaults = {
+            unit = "percent";
+            min = 0;
+            max = 100;
+            color.mode = "palette-classic";
+            custom = {
+              lineWidth = 2;
+              fillOpacity = 20;
+              spanNulls = true;
+            };
+          };
+          overrides = [ ];
+        };
+      }
+      {
+        id = 13;
+        type = "timeseries";
+        title = "Boot Drive Partitions";
+        gridPos = {
+          h = 8;
+          w = 12;
+          x = 12;
+          y = 32;
+        };
+        datasource = promDs;
+        targets = [
+          {
+            datasource = promDs;
+            expr = "partition_used_bytes{mount=\"/boot\"} / partition_size_bytes{mount=\"/boot\"} * 100";
+            legendFormat = "/boot";
+            refId = "A";
+          }
+          {
+            datasource = promDs;
+            expr = "partition_used_bytes{mount=\"/persistent\"} / partition_size_bytes{mount=\"/persistent\"} * 100";
+            legendFormat = "/persistent";
+            refId = "B";
+          }
+          {
+            datasource = promDs;
+            expr = "partition_used_bytes{mount=\"/nix\"} / partition_size_bytes{mount=\"/nix\"} * 100";
+            legendFormat = "/nix";
+            refId = "C";
+          }
+        ];
+        fieldConfig = {
+          defaults = {
+            unit = "percent";
+            min = 0;
+            max = 100;
+            color.mode = "palette-classic";
+            custom = {
+              lineWidth = 2;
+              fillOpacity = 20;
+              spanNulls = true;
+            };
+          };
+          overrides = [ ];
+        };
+      }
     ];
   };
 in
@@ -875,6 +974,24 @@ in
     };
   };
 
+  # -- Disk/pool usage textfile collector --
+  systemd.services.disk-usage-collector = {
+    description = "Collect ZFS pool and partition usage metrics for Prometheus";
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = lib.getExe diskUsageCollector;
+    };
+    environment.TEXTFILE = "${textfileDir}/disk-usage.prom";
+  };
+
+  systemd.timers.disk-usage-collector = {
+    wantedBy = [ "timers.target" ];
+    timerConfig = {
+      OnCalendar = "*:*:0/60"; # every 60 seconds
+      RandomizedDelaySec = "10s";
+    };
+  };
+
   systemd.tmpfiles.rules = [
     "d ${textfileDir} 0755 root root -"
   ];