feat(payments): implement server-side price resolution for subscription payments

Add a new function `resolve_fiat_offer_price_for_user` to determine the correct offer price for users based on their subscription choices and any applicable discounts. Update payment handlers for CryptoPay, FreeKassa, Platega, SeverPay, and YooKassa to utilize this function, ensuring callback price validation and improving error handling for price mismatches. This enhances security by preventing callback payload tampering and ensures accurate pricing for users.

bot/handlers/user/subscription/payments_subscription.py

@@ -12,6 +12,36 @@ from config.settings import Settings
 router = Router(name="user_subscription_payments_selection_router")
 
 
+async def resolve_fiat_offer_price_for_user(
+    session: AsyncSession,
+    settings: Settings,
+    user_id: int,
+    months: float,
+    sale_mode: str,
+    promo_code_service=None,
+) -> Optional[float]:
+    """Resolve offer price server-side to prevent callback payload tampering."""
+    price_source = (
+        getattr(settings, "traffic_packages", {}) or {}
+        if sale_mode == "traffic"
+        else (settings.subscription_options or {})
+    )
+    base_price = price_source.get(months)
+    if base_price is None:
+        return None
+
+    resolved_price = float(base_price)
+    if promo_code_service:
+        active_discount_info = await promo_code_service.get_user_active_discount(session, user_id)
+        if active_discount_info:
+            discount_pct, _ = active_discount_info
+            resolved_price, _ = promo_code_service.calculate_discounted_price(
+                resolved_price,
+                discount_pct,
+            )
+    return resolved_price
+
+
 @router.callback_query(F.data.startswith("subscribe_period:"))
 async def select_subscription_period_callback_handler(
     callback: types.CallbackQuery,