refactor(promo): streamline discount consumption logic

Simplify the discount consumption process in the PromoCodeService by removing unnecessary checks for active discounts and improving logging for various scenarios. Ensure that successful payments are always accounted for, even if the associated reservation has expired. This enhances clarity and reliability in the discount application workflow.

bot/services/stars_service.py

@@ -58,17 +58,28 @@ class StarsService:
     async def create_invoice(self, session: AsyncSession, user_id: int, months: float,
                              stars_price: int, description: str, sale_mode: str = "subscription",
                              promo_code_service=None) -> Optional[int]:
-        # Always resolve base price server-side to avoid trusting callback payload.
+        # Always resolve base price server-side and reject unknown packages.
         resolved_base_price = self._resolve_base_stars_price(months, sale_mode)
         if resolved_base_price is None:
             logging.warning(
-                "Stars base price not found for sale_mode=%s months=%s, fallback to callback price.",
+                "Stars invoice rejected: base price not found for sale_mode=%s months=%s.",
+                sale_mode,
+                months,
+            )
+            return None
+
+        original_stars_price = int(resolved_base_price)
+
+        # Detect callback tampering (or stale callback payload) and prefer server-side price.
+        if int(stars_price) != original_stars_price:
+            logging.warning(
+                "Stars callback price mismatch for user %s: callback=%s, resolved=%s, sale_mode=%s, months=%s",
+                user_id,
+                stars_price,
+                original_stars_price,
                 sale_mode,
                 months,
             )
-            original_stars_price = stars_price
-        else:
-            original_stars_price = int(resolved_base_price)
 
         # Invoice amount starts from the base price and discount is applied once.
         stars_price = original_stars_price