[add]: Loan details and repayment fix

app/models/customer.py

@@ -20,11 +20,11 @@ class Customer(db.Model):
     )
 
     @classmethod
-    def is_eligible(cls, customer_id):
+    def is_valid_customer(cls, customer_id):
         customer = cls.query.filter_by(id=customer_id).first()
         if not customer:
-            return False, "Customer not found"
-        return True, "Customer is eligible"
+            return False
+        return True
 
     @classmethod
     def create_customer(cls, id, msisdn, country_code, account_id, account_type='savings'):