Fix DMS entity assignment and Geschichte document linking

- DMS edit view: add Destinatär, Land, Pächter, Verpachtung dropdowns
  so documents can be assigned to entities after upload
- Geschichte: add M2M dokumente field on GeschichteSeite model
- Geschichte form: checkboxes to select/link Stiftungsgeschichte docs
- Geschichte detail: show linked documents in sidebar with download

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

app/stiftung/views/dms.py

@@ -247,13 +247,34 @@ def dms_edit(request, pk):
         dok.titel = request.POST.get("titel", dok.titel).strip()[:255]
         dok.beschreibung = request.POST.get("beschreibung", "").strip()
         dok.kontext = request.POST.get("kontext", dok.kontext)
+
+        # Entity assignments
+        dest_id = request.POST.get("destinataer_id", "").strip()
+        land_id = request.POST.get("land_id", "").strip()
+        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.verpachtung_id = verp_id if verp_id else None
+
         dok.save()
         dok.update_suchvektor()
         messages.success(request, "Metadaten gespeichert.")
         return redirect("stiftung:dms_detail", pk=dok.pk)
 
+    destinataere = Destinataer.objects.filter(aktiv=True).order_by("nachname", "vorname")
+    laendereien = Land.objects.filter(aktiv=True).order_by("lfd_nr")
+    paechter_qs = Paechter.objects.filter(aktiv=True).order_by("nachname")
+    verpachtungen = LandVerpachtung.objects.select_related("land", "paechter").order_by("-vertragsbeginn")[:50]
+
     context = {
         "dok": dok,
         "kontext_choices": DokumentDatei.KONTEXT_CHOICES,
+        "destinataere": destinataere,
+        "laendereien": laendereien,
+        "paechter_qs": paechter_qs,
+        "verpachtungen": verpachtungen,
     }
     return render(request, "stiftung/dms/edit.html", context)