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/nl.ts

@@ -743,6 +743,7 @@ const nl: Record<string, string> = {
   'reservations.pendingSave': 'wordt opgeslagen…',
   'reservations.uploading': 'Uploaden...',
   'reservations.attachFile': 'Bestand bijvoegen',
+  'reservations.linkExisting': 'Bestaand bestand koppelen',
   'reservations.toast.saveError': 'Opslaan mislukt',
   'reservations.toast.updateError': 'Bijwerken mislukt',
   'reservations.toast.deleteError': 'Verwijderen mislukt',