spark-wallet: use onionServices

Also remove the unneeded definition of ReadWritePaths because the
service doesn't need write access to onion files.

modules/onion-services.nix

@@ -103,6 +103,15 @@ in {
     # Set sensible defaults for some services
     {
       nix-bitcoin.onionServices = {
+        spark-wallet = {
+          externalPort = 80;
+          # Enable 'public' by default, but don't auto-enable the onion service.
+          # When the onion service is enabled, 'public' lets spark-wallet generate
+          # a QR code for accessing the web interface.
+          public = true;
+          # Low priority so we can override this with mkDefault in ./presets/enable-tor.nix
+          enable = mkOverride 1400 false;
+        };
         btcpayserver = {
           externalPort = 80;
         };