Setup binary cache on c3 and optimize nix settings.

common/global/nix.nix

@@ -1,8 +1,30 @@
 {
-  nix.settings.trusted-users = [
-    "root"
-    "@wheel"
-  ];
+  nix.settings = {
+    trusted-users = [
+      "root"
+      "@wheel"
+    ];
+
+    # Binary cache configuration
+    substituters = [
+      "http://c3:5000"  # Local cluster cache on c3
+      "https://cache.nixos.org"
+    ];
+    trusted-public-keys = [
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+      # TODO: Add c3 cache public key here after first deploy of c3
+      # Get it with: ssh c3 cat /persist/nix-cache/cache-pub-key.txt
+      # "c3-cache:..."
+    ];
+
+    # Performance tuning
+    max-jobs = "auto";           # Use all cores for parallel builds
+    cores = 0;                   # Each build can use all cores
+    max-substitution-jobs = 16;  # Faster fetching from caches
+    http-connections = 25;       # More parallel downloads
+    download-attempts = 3;       # Retry failed downloads
+  };
+
   nix.gc = {
     automatic = true;
     dates = "weekly";