feat(MemberOrder): add budget field to MemberOrder and MemberOrderHistory models

- Introduced a new budget field in both MemberOrder and MemberOrderHistory models to track budget amounts.
- Updated memberOrderService to handle budget in serialization and normalization processes.
- Enhanced OrdersPanel component to include budget input and display in the UI.
- Added German localization for the new budget term to ensure consistency across languages.

backend/migrations/20260421_add_budget_to_member_orders.sql

@@ -0,0 +1,5 @@
+ALTER TABLE `member_orders`
+  ADD COLUMN `budget` DECIMAL(10,2) NOT NULL DEFAULT 0.00 AFTER `paid_amount`;
+
+ALTER TABLE `member_order_history`
+  ADD COLUMN `budget` DECIMAL(10,2) NOT NULL DEFAULT 0.00 AFTER `paid_amount`;