heavy refactor

src/constants.py

@@ -0,0 +1,16 @@
+from enum import Enum
+
+IBAN_PATTERN = r"^[A-Z]{2}\d{2}[A-Z0-9]{11,30}$"
+
+
+class Currency(str, Enum):
+    EUR = "EUR"
+    USD = "USD"
+    GBP = "GBP"
+    CHF = "CHF"
+
+    def __str__(self):
+        return self.value
+
+    def __repr__(self):
+        return f"Currency.{self.name}"