fix: traffic addon price mismatch — keyboard showed prorated, handler charged full month

Keyboard calculated: price * days_remaining / 30 (true proration)
Handler calculated: price * max(30, days_remaining) / 30 (always >= 30 days)

With 17 days remaining: keyboard showed 84₽, handler charged 149₽.

Fix: change calculate_prorated_price default min_charge_days from 30 to 1.
Now all callers (traffic, countries, servers, miniapp, auto-purchase)
use true proration matching the displayed price.

app/utils/pricing_utils.py

@@ -20,7 +20,7 @@ def calculate_months_from_days(days: int) -> int:
     return max(1, round(days / 30))
 
 
-def calculate_prorated_price(monthly_price: int, end_date: datetime, min_charge_days: int = 30) -> tuple[int, int]:
+def calculate_prorated_price(monthly_price: int, end_date: datetime, min_charge_days: int = 1) -> tuple[int, int]:
     """Calculate prorated price based on remaining days.
 
     Returns: