Fix email-Destinatär document linking and add email delete

- When manually assigning an email to a Destinatär, also update
  associated DokumentLink records so attachments appear in the
  Destinatär's Dokumente tab
- Add email delete functionality (view, URL, buttons in list and detail)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/stiftung/views/__init__.py

@@ -89,6 +89,7 @@ from .geschichte import (  # noqa: F401
     kalender_api_events,
     email_eingang_list,
     email_eingang_detail,
+    email_eingang_delete,
     email_eingang_poll_trigger,
 )