feat: implement basic Letter model and unit tests

backend/letters/models.py

@@ -1 +1,26 @@
-# Create your models here.
+import uuid
+
+from django.conf import settings
+from django.db import models
+
+
+class Letter(models.Model):
+    class Type(models.TextChoices):
+        KEPT = "KEPT", "Kept"
+        SENT = "SENT", "Sent"
+        VAULT = "VAULT", "Vault"
+
+    class Status(models.TextChoices):
+        DRAFT = "DRAFT", "Draft"
+        SEALED = "SEALED", "Sealed"
+        BURNED = "BURNED", "Burned"
+
+    public_id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name="letters")
+    type = models.CharField(max_length=10, choices=Type.choices, default=Type.KEPT)
+    status = models.CharField(max_length=10, choices=Status.choices, default=Status.DRAFT)
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    def __str__(self):
+        return f"{self.type} - {self.status}"