v4.1.0: DMS email documents, category-specific Nachweis linking, version system

- Save cover email body as DMS document with new 'email' context type
- Show email body separately from attachments in email detail view
- Add per-category DMS document assignment in quarterly confirmation
  (Studiennachweis, Einkommenssituation, Vermögenssituation)
- Add VERSION file and context processor for automatic version display
- Add MCP server, agent system, import/export, and new migrations
- Update compose files and production environment template

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

app/stiftung/views/dms.py

@@ -254,9 +254,9 @@ def dms_edit(request, pk):
         paechter_id = request.POST.get("paechter_id", "").strip()
         verp_id = request.POST.get("verpachtung_id", "").strip()
 
-        dok.destinataer_id = int(dest_id) if dest_id else None
-        dok.land_id = int(land_id) if land_id else None
-        dok.paechter_id = int(paechter_id) if paechter_id else None
+        dok.destinataer_id = dest_id if dest_id else None
+        dok.land_id = land_id if land_id else None
+        dok.paechter_id = paechter_id if paechter_id else None
         dok.verpachtung_id = verp_id if verp_id else None
 
         dok.save()