feat: remove postal code validation

docs/database_schema.dbml

@@ -188,7 +188,7 @@ Table members {
     - email: 5-254 characters, valid email format (required)
     - join_date: cannot be in future
     - exit_date: must be after join_date (if both present)
-    - postal_code: exactly 5 digits (if present)
+    - postal_code: optional (no format validation)
   '''
 }