feat: add notification field for vault letters and helper tasks

backend/letters/migrations/0008_letter_notified_at.py

@@ -0,0 +1,17 @@
+# Generated by Django 6.0.4 on 2026-04-17 07:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("letters", "0007_alter_letter_public_id"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="letter",
+            name="notified_at",
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]