Implement buy flow and migrate DTOs to schemas

Move DTO definitions from dto/ to schemas/ and update imports.
Add buy handler with FSM state management for device selection,
whitelist toggling, and duration choice. Include pricing logic,
inline keyboards, and new configuration fields.

handlers/menus.py

@@ -4,7 +4,7 @@ from aiogram.types import CallbackQuery, Message
 from aiogram.fsm.context import FSMContext
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from dto.di import DependenciesDTO
+from schemas.di import DependenciesDTO
 from keyboards.client import main_menu
 from texts import build_main_menu_text