migrate git URLs from git.t-juice.club to code.t-juice.club

Update all flake URLs to use the new Forgejo instance. This includes
auto-upgrade, nixos-rebuild-test, homelab-deploy listener, nixos-exporter,
nix-cache02 builder, and the bootstrap script.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

system/nix.nix

@@ -14,7 +14,7 @@ let
       branch="$2"
       shift 2
 
-      exec nixos-rebuild "$action" --flake "git+https://git.t-juice.club/torjus/nixos-servers.git?ref=$branch" "$@"
+      exec nixos-rebuild "$action" --flake "git+https://code.t-juice.club/torjus/nixos-servers.git?ref=$branch" "$@"
     '';
   };
 in