init: syncthing module

Assumes one `introducer` peer, which can AutoShare configured folders,
with peers that know the `deviceID` of the introducer.

checks/flake-module.nix

@@ -18,6 +18,7 @@
           deltachat = import ./deltachat nixosTestArgs;
           meshnamed = import ./meshnamed nixosTestArgs;
           borgbackup = import ./borgbackup nixosTestArgs;
+          syncthing = import ./syncthing nixosTestArgs;
         };
         schemaTests = pkgs.callPackages ./schemas.nix {
           inherit self;