Refactor subscription duration calculation in PanelWebhookService and SubscriptionService

- Introduced a utility function `add_months` to handle subscription duration calculations based on calendar months instead of a fixed 30-day period.
- Updated the auto-renewal logic in `PanelWebhookService` to use the new function for extending subscription end dates.
- Adjusted the duration calculation in `SubscriptionService` to derive the end date after a specified number of months, improving accuracy in subscription management.

bot/utils/date_utils.py

@@ -0,0 +1,27 @@
+from datetime import datetime, timedelta
+
+
+def add_months(base_dt: datetime, months_to_add: int) -> datetime:
+    """Add calendar months to a datetime, clamping the day to the month's length.
+
+    Preserves tzinfo from base_dt.
+    """
+    year = base_dt.year
+    month = base_dt.month + months_to_add
+    day = base_dt.day
+
+    # Normalize year and month
+    year += (month - 1) // 12
+    month = ((month - 1) % 12) + 1
+
+    # Determine last day of target month by rolling to next month's first day and subtracting 1 day
+    if month == 12:
+        next_month_first = datetime(year + 1, 1, 1, tzinfo=base_dt.tzinfo)
+    else:
+        next_month_first = datetime(year, month + 1, 1, tzinfo=base_dt.tzinfo)
+    last_day = (next_month_first - timedelta(days=1)).day
+
+    clamped_day = min(day, last_day)
+    return base_dt.replace(year=year, month=month, day=clamped_day)
+
+