fix: add missing postal_code in seeds

postal_code is a Vereinfacht-required field. When Vereinfacht is
configured, seeds failed for members without postal_code.

priv/repo/seeds.exs

@@ -328,6 +328,7 @@ member_attrs_list = [
     city: "Berlin",
     street: "Kastanienallee",
     house_number: "8",
+    postal_code: "10435",
     membership_fee_type_id: Enum.at(all_fee_types, 2).id,
     cycle_status: :mixed
   },
@@ -338,7 +339,8 @@ member_attrs_list = [
     join_date: ~D[2022-11-10],
     city: "Berlin",
     street: "Kastanienallee",
-    house_number: "8"
+    house_number: "8",
+    postal_code: "10435"
     # No membership_fee_type_id - member without fee type
   }
 ]