feat(nix): add harmonia cache to nix.settings

Configure all NixOS machines to use the internal harmonia binary cache:
- Add nix-cache.johnogle.info as substituter
- Add harmonia signing public key to trusted-public-keys
- Enable fallback for local builds when cache unreachable
- Set 5s connect-timeout for faster fallback

Refs: x-qdkuu

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

renovate.json

@@ -13,7 +13,7 @@
   "lockFileMaintenance": {
     "enabled": true,
     "schedule": [
-      "before 5am on monday"
+      "after 2pm and before 4pm on Saturday"
     ]
   },
   "dependencyDashboard": true,
@@ -37,6 +37,9 @@
         "/^nixpkgs$/",
         "/^home-manager$/",
         "/^nix-darwin$/"
+      ],
+      "schedule": [
+        "after 2pm and before 4pm on Saturday"
       ]
     },
     {
@@ -48,6 +51,21 @@
       "matchPackageNames": [
         "/nixpkgs-unstable/",
         "/home-manager-unstable/"
+      ],
+      "schedule": [
+        "after 2pm and before 4pm on Saturday"
+      ]
+    },
+    {
+      "description": "nixpkgs-qt updates on Saturday (staggered from main ecosystem)",
+      "matchManagers": [
+        "nix"
+      ],
+      "matchPackageNames": [
+        "/nixpkgs-qt/"
+      ],
+      "schedule": [
+        "after 4pm and before 6pm on Saturday"
       ]
     },
     {

roles/common.nix

@@ -23,7 +23,13 @@
         max-jobs = "auto";
         trusted-users = [ "johno" ];
         substituters = [
+          "https://nix-cache.johnogle.info"
         ];
+        trusted-public-keys = [
+          "nix-cache.johnogle.info-1:IC5x8BxnrqkU9XqhMdDnZLtSg9Y3rBJVXhve5DJ92J0="
+        ];
+        fallback = true;
+        connect-timeout = 5;
       };
 
       gc = {