feat: add encrypted_dek field to Letter model and enforce its presence during serialization

backend/letters/migrations/0005_letter_encrypted_dek.py

@@ -0,0 +1,17 @@
+# Generated by Django 6.0.4 on 2026-04-11 12:56
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("letters", "0004_letter_burned_at_letter_opened_at_letter_sealed_at"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="letter",
+            name="encrypted_dek",
+            field=models.TextField(blank=True, null=True),
+        ),
+    ]