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.

middlewares/di.py

@@ -3,7 +3,7 @@ from aiogram import BaseMiddleware
 from aiogram.types import TelegramObject
 from sqlalchemy.ext.asyncio import async_sessionmaker
 
-from dto.di import DependenciesDTO
+from schemas.di import DependenciesDTO
 
 
 class DIMiddleware(BaseMiddleware):