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/models/destinataere.py

@@ -26,7 +26,20 @@ class Destinataer(models.Model):
         ("andere", "Andere"),
     ]
 
+    ANREDE_CHOICES = [
+        ("Herr", "Herr"),
+        ("Frau", "Frau"),
+        ("Divers", "Divers"),
+    ]
+
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    anrede = models.CharField(
+        max_length=20,
+        choices=ANREDE_CHOICES,
+        blank=True,
+        null=True,
+        verbose_name="Anrede",
+    )
     familienzweig = models.CharField(
         max_length=100, choices=FAMILIENZWIG_CHOICES, blank=True, null=True
     )