refactor: remove _serialized field and implement efficient vars selection

- Remove _serialized field from vars interface to prevent serialization
  errors with throwing passBackend field
- Implement direct selection of generator fields using multi-select syntax
- Refactor vars_generators() to use new Generator.from_flake() method that
  selects only safe fields (avoiding non-serializable values)
- Remove unused legacy methods: Generator.from_json(), Var.from_json(),
  Prompt.from_json()
- Update precaching to match new selection approach

This fixes the serialization errors that were preventing vars from working
with the new password-store implementation by avoiding the problematic
_serialized field entirely.

nixosModules/clanCore/vars/interface.nix

@@ -34,50 +34,6 @@ let
 in
 {
   options = {
-    _serialized = lib.mkOption {
-      readOnly = true;
-      internal = true;
-      description = ''
-        JSON serialization of the generators.
-        This is read from the python client to generate the specified resources.
-      '';
-      default = {
-        # TODO: We don't support per-machine choice of backends
-        # Configuring different backend doesn't work, this information should be made read only and configured
-        # Via clan.settings instead.
-        inherit (config.settings) secretModule publicModule;
-        # Serialize generators, so that we can use them in the python client
-        # This need to be done because we have some non-serializable values in the generators
-        # Like the finalScript (derivation) or pkgs.
-        generators = lib.flip lib.mapAttrs config.generators (
-          _name: generator: {
-            inherit (generator)
-              name
-              dependencies
-              validationHash
-              migrateFact
-              share
-              prompts
-              ;
-
-            files = lib.flip lib.mapAttrs generator.files (
-              _name: file: {
-                inherit (file)
-                  name
-                  owner
-                  group
-                  mode
-                  deploy
-                  secret
-                  neededFor
-                  ;
-              }
-            );
-          }
-        );
-      };
-    };
-
     settings = import ./settings-opts.nix { inherit lib; };
     generators = lib.mkOption {
       description = ''