refactor: split backend by context instead of type 'controllers/models/...'

app/user/controllers/theme_controller.ts

@@ -0,0 +1,12 @@
+import { PREFER_DARK_THEME } from '#user/constants/theme';
+import { updateThemeValidator } from '#user/validators/update_theme_validator';
+import type { HttpContext } from '@adonisjs/core/http';
+
+export default class ThemeController {
+	async index({ request, session, response }: HttpContext) {
+		const { preferDarkTheme } =
+			await request.validateUsing(updateThemeValidator);
+		session.put(PREFER_DARK_THEME, preferDarkTheme);
+		return response.ok({ message: 'ok' });
+	}
+}