feat: implement letter encryption fields, lifecycle timestamps, and API endpoints with validation

backend/letters/migrations/0003_letter_unlock_at.py

@@ -0,0 +1,17 @@
+# Generated by Django 6.0.4 on 2026-04-10 15:47
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("letters", "0002_letter_encrypted_content_letter_encrypted_metadata"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="letter",
+            name="unlock_at",
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]