Fix Paperless thumbnail URLs: Remove duplicate /paperless/ path

- get_paperless_thumbnail_url() was adding /paperless/ twice
- PAPERLESS_API_URL already includes /paperless/ prefix
- Now generates: base_url/api/documents/{id}/thumb/ instead of base_url/api/paperless/documents/{id}/thumb/
- Fixes thumbnail display functionality

app/stiftung/models.py

@@ -1314,7 +1314,7 @@ class DokumentLink(models.Model):
         from django.conf import settings
 
         if settings.PAPERLESS_API_URL:
-            return f"{settings.PAPERLESS_API_URL}/api/paperless/documents/{self.paperless_document_id}/thumb/"
+            return f"{settings.PAPERLESS_API_URL}/api/documents/{self.paperless_document_id}/thumb/"
         return None
 
     def get_verpachtung(self):