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

backend/letters/serializers.py

@@ -0,0 +1,24 @@
+from rest_framework import serializers
+
+from .models import Letter
+
+
+class LetterSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Letter
+        fields = [
+            "public_id",
+            "type",
+            "status",
+            "encrypted_content",
+            "encrypted_metadata",
+            "unlock_at",
+            "sealed_at",
+            "created_at",
+            "updated_at",
+        ]  # user to be fetched from request
+        read_only_fields = ["public_id", "created_at", "updated_at"]
+
+    def create(self, validated_data):
+        user = self.context["request"].user  # get user from access token
+        return Letter.objects.create(user=user, **validated_data)