test: add simplified file permissions test

Add a test that validates file permissions, ownership, and service access
for key Nixarr services. The test verifies:

- Users and groups exist with correct membership
- Directory ownership matches expected patterns
- Services can write/read files in media directories
- Cross-seed can access transmission state
- fix-permissions command works correctly

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

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

flake.nix

@@ -34,6 +34,13 @@
   in {
     nixosModules.default.imports = [./nixarr vpnconfinement.nixosModules.default];
 
+    # Add tests attribute to the flake outputs
+    checks = forAllSystems ({pkgs}: {
+      permissions-test = pkgs.callPackage ./tests/permissions-test.nix {
+        inherit (self) nixosModules;
+      };
+    });
+
     devShells = forAllSystems ({pkgs}: {
       default = pkgs.mkShell {
         packages = with pkgs; [