Remove iscan/epkowa dependency to fix ISO build

- Remove epkowa from SANE extraBackends due to broken iscan package
- Keep sane-airscan for network scanner support
- Resolves ISO build failures from iscan scanner bundle errors

🤖 Generated with [Claude Code](https://claude.ai/code)

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

modules/hardware/default.nix

@@ -52,7 +52,8 @@ with lib;
       enable = true;
       extraBackends = with pkgs; [
         sane-airscan
-        epkowa
+        # epkowa removed due to iscan build issues
+        # Use epsonscan2 or imagescan for Epson scanner support instead
       ];
     };