refactor: handle exception on vault unlock date check for validation

backend/letters/migrations/0009_alter_letter_notified_at_alter_letter_unlock_at.py

@@ -0,0 +1,22 @@
+# Generated by Django 6.0.4 on 2026-04-17 18:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("letters", "0008_letter_notified_at"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="letter",
+            name="notified_at",
+            field=models.DateTimeField(blank=True, db_index=True, null=True),
+        ),
+        migrations.AlterField(
+            model_name="letter",
+            name="unlock_at",
+            field=models.DateTimeField(blank=True, db_index=True, null=True),
+        ),
+    ]