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.

spec/requests/api/v1/users_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require 'swagger_helper'
+
+RSpec.describe 'API V1 Users', type: :request do
+  path '/api/v1/user/enable_ai' do
+    patch 'Enable AI features for the authenticated user' do
+      tags 'Users'
+      consumes 'application/json'
+      produces 'application/json'
+      security [{ apiKeyAuth: [] }]
+
+      response '200', 'ai enabled' do
+        schema type: :object,
+               properties: {
+                 user: {
+                   type: :object,
+                   properties: {
+                     id: { type: :string, format: :uuid },
+                     email: { type: :string },
+                     first_name: { type: :string, nullable: true },
+                     last_name: { type: :string, nullable: true },
+                     ui_layout: { type: :string, enum: %w[dashboard intro] },
+                     ai_enabled: { type: :boolean }
+                   }
+                 }
+               }
+        run_test!
+      end
+
+      response '401', 'unauthorized' do
+        schema '$ref' => '#/components/schemas/ErrorResponse'
+        run_test!
+      end
+    end
+  end
+end