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/migrations/0053_geschichte_dokumente_m2m.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2026-03-12 10:40
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0052_alter_dokumentdatei_kontext_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='geschichteseite',
+            name='dokumente',
+            field=models.ManyToManyField(blank=True, related_name='geschichte_seiten', to='stiftung.dokumentdatei', verbose_name='Verknüpfte Dokumente'),
+        ),
+    ]