[add]: loan_repayment event

app/models/loan.py

@@ -26,6 +26,16 @@ class Loan(db.Model):
             return False, "Customer has active loans"
         return True, "No active loans"
     
+    @classmethod
+    def get_customer_loan(cls, loan_id, customer_id):
+        """
+        Check if a loan with the given ID exists and if the loan belongs to the specified customer_id.
+        """
+        loan = cls.query.filter_by(id=loan_id, customer_id=customer_id).first()
+        if not loan:
+            raise ValueError(f"Loan with ID {loan_id} does not exist or does not belong to customer {customer_id}.")
+        return loan
+    
     
     def __repr__(self):
         return f'<Loan {self.id}>'