Move AI enable endpoint to user scope

Replace /api/v1/auth/enable_ai with /api/v1/user/enable_ai by moving the action into Api::V1::UsersController, updating Flutter client calls, tests, and OpenAPI docs.

app/controllers/api/v1/users_controller.rb

@@ -0,0 +1,28 @@
+module Api
+  module V1
+    class UsersController < BaseController
+      def enable_ai
+        user = current_resource_owner
+
+        if user.update(ai_enabled: true)
+          render json: { user: mobile_user_payload(user) }
+        else
+          render json: { errors: user.errors.full_messages }, status: :unprocessable_entity
+        end
+      end
+
+      private
+
+        def mobile_user_payload(user)
+          {
+            id: user.id,
+            email: user.email,
+            first_name: user.first_name,
+            last_name: user.last_name,
+            ui_layout: user.ui_layout,
+            ai_enabled: user.ai_enabled?
+          }
+        end
+    end
+  end
+end