grafana: use instant queries for fleet dashboard stat panels

Prevents stat panels from being affected by dashboard time range selection.

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

services/grafana/dashboards/nixos-fleet.json

@@ -35,7 +35,8 @@
         {
           "expr": "count(nixos_flake_revision_behind{tier=~\"$tier\"} == 1)",
           "legendFormat": "Behind",
-          "refId": "A"
+          "refId": "A",
+          "instant": true
         }
       ],
       "fieldConfig": {
@@ -69,7 +70,8 @@
         {
           "expr": "count(nixos_config_mismatch{tier=~\"$tier\"} == 1)",
           "legendFormat": "Need Reboot",
-          "refId": "A"
+          "refId": "A",
+          "instant": true
         }
       ],
       "fieldConfig": {
@@ -103,7 +105,8 @@
         {
           "expr": "count(nixos_flake_info{tier=~\"$tier\"})",
           "legendFormat": "Hosts",
-          "refId": "A"
+          "refId": "A",
+          "instant": true
         }
       ],
       "fieldConfig": {
@@ -130,7 +133,8 @@
         {
           "expr": "max(nixos_flake_input_age_seconds{input=\"nixpkgs\", tier=~\"$tier\"})",
           "legendFormat": "Nixpkgs",
-          "refId": "A"
+          "refId": "A",
+          "instant": true
         }
       ],
       "fieldConfig": {
@@ -164,7 +168,8 @@
         {
           "expr": "count(nixos_flake_revision_behind{tier=~\"$tier\"} == 0)",
           "legendFormat": "Up-to-date",
-          "refId": "A"
+          "refId": "A",
+          "instant": true
         }
       ],
       "fieldConfig": {