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.

misc/utils.py

@@ -1,7 +1,9 @@
 from typing import Optional
 from random import choice
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 
+from config import settings
+from schemas.subscriptions import SubscriptionDuration, SubscriptionPlan
 import texts
 
 QUOTES: list[str] = [
@@ -125,3 +127,45 @@ def format_username(username: Optional[str]) -> str:
     if username:
         return "@" + username
     return texts.NO_USERNAME
+
+
+def convert_duration_to_int(duration: SubscriptionDuration) -> int:
+    mapping: dict[str, int] = {
+        SubscriptionDuration.MONTH.value: 1,
+        SubscriptionDuration.THREE_MONTHS.value: 3,
+        SubscriptionDuration.SIX_MONTHS.value: 6,
+        SubscriptionDuration.YEAR.value: 12,
+    }
+    return mapping.get(duration.value, 1)
+
+
+def convert_int_to_duration(value: int) -> SubscriptionDuration:
+    mapping: dict[int, SubscriptionDuration] = {
+        1: SubscriptionDuration.MONTH,
+        3: SubscriptionDuration.THREE_MONTHS,
+        6: SubscriptionDuration.SIX_MONTHS,
+        12: SubscriptionDuration.YEAR,
+    }
+    return mapping.get(value, SubscriptionDuration.MONTH)
+
+
+def convert_duration_to_timedelta(duration: SubscriptionDuration) -> timedelta:
+    MONTH_DELTA = timedelta(days=30)
+    mapping: dict[str, timedelta] = {
+        SubscriptionDuration.MONTH.value: MONTH_DELTA,
+        SubscriptionDuration.THREE_MONTHS.value: MONTH_DELTA * 3,
+        SubscriptionDuration.SIX_MONTHS.value: MONTH_DELTA * 6,
+        SubscriptionDuration.YEAR.value: MONTH_DELTA * 12,
+    }
+    return mapping.get(duration.value, MONTH_DELTA)
+
+
+def calculate_price(subscription: SubscriptionPlan):
+    return (
+        (subscription.devices * settings.per_device_cost)
+        + (
+            int(subscription.whitelists) * settings.whitelist_cost
+            if subscription.devices < settings.whitelist_device_threshold
+            else 0
+        )
+    ) * convert_duration_to_int(subscription.duration)