feat(cache): setup nix-serve

Nix-serve is [a cache server](https://github.com/edolstra/nix-serve)
that sets up your nix store to be served as a binary cache. As we're
not going to be using cachix anymore as we are on tangled, we need to do
this to have a cache

packetmix/systems/midnight/cache.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  networking.firewall.allowedTCPPorts = [ 1025 ];
+
+  services.nix-serve = {
+    enable = true;
+    secretKeyFile = "/secrets/cache/signer.key";
+
+    bindAddress = "0.0.0.0";
+    port = 1025;
+  };
+}

packetmix/systems/midnight/secrets.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  clicks.storage.impermanence.persist.directories = [ "/secrets" ];
+}

packetmix/systems/teal/cache.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  services.nginx.enable = true;
+  services.nginx.virtualHosts."cache.freshlybakedca.ke" = {
+    addSSL = true;
+    enableACME = true;
+    acmeRoot = null;
+
+    locations."/" = {
+      proxyPass = "http://midnight:1025";
+      recommendedProxySettings = true;
+      proxyWebsockets = true;
+    };
+  };
+}