Add Anrede field to Destinatär model (STI-86)

Adds optional salutation (Herr/Frau/Divers) to the Destinatär model
with migration, form support, admin integration and template display.

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

app/stiftung/migrations/0063_add_anrede_to_destinataer.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2026-03-21 21:45
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0062_veranstaltungseinladung_vorlage'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='destinataer',
+            name='anrede',
+            field=models.CharField(blank=True, choices=[('Herr', 'Herr'), ('Frau', 'Frau'), ('Divers', 'Divers')], max_length=20, null=True, verbose_name='Anrede'),
+        ),
+    ]