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/__init__.py

@@ -1,4 +1,5 @@
 from .menus import router as menus_router
 from .support import router as support_router
+from .buy import router as buy_router
 
-routers = [menus_router, support_router]
+routers = [menus_router, support_router, buy_router]