feat: multi-link files to multiple bookings and places — closes #23

Files can now be linked to multiple bookings and places simultaneously
via a new file_links junction table. Booking modal includes a file picker
to link existing uploads. Unlinking removes the association without
deleting the file.

client/src/i18n/translations/zh.ts

@@ -743,6 +743,7 @@ const zh: Record<string, string> = {
   'reservations.pendingSave': '将被保存…',
   'reservations.uploading': '上传中...',
   'reservations.attachFile': '附加文件',
+  'reservations.linkExisting': '关联已有文件',
   'reservations.toast.saveError': '保存失败',
   'reservations.toast.updateError': '更新失败',
   'reservations.toast.deleteError': '删除失败',