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.

config.py

@@ -13,6 +13,11 @@ class settings(BaseSettings):
     admin_group_id: int = Field(alias="ADMIN_GROUP_ID")
     remnawave_url: str = Field(alias="REMNAWAVE_URL")
     remnawave_token: str = Field(alias="REMNAWAVE_TOKEN")
+    min_devices: int = Field(3, alias="MIN_DEVICES")
+    max_devices: int = Field(12, alias="MAX_DEVICES")
+    per_device_cost: int = Field(50, alias="PER_DEVICE_COST")
+    whitelist_cost: int = Field(100, alias="WHITELIST_COST")
+    whitelist_device_threshold: int = Field(6, alias="WHITELIST_DEVICE_THRESHOLD")
 
 
 settings = settings()  # type: ignore