feat: restricted excuse to be 1 per day

Co-authored-by: Copilot <copilot@github.com>

Author: fatmaebrahim

server/cshr/utils/balance_calculator.py

@@ -153,6 +153,11 @@ def check_requests_in_selected_dates(self, is_update: bool = False):
         if is_update:
             requests = requests.exclude(id=self.vacation.id)
 
+        if self.vacation.reason == "excuse":
+            existing_excuses = requests.filter(reason="excuse", from_date__date=self.vacation.from_date.date())
+            if existing_excuses.exists():
+                raise ValueError("There is already an excuse on the selected date.")
+
         if requests.exists():
             grammar = "is a request" if len(requests) == 1 else "are requests"
             raise ValueError(f"There {grammar} on the selected dates.")