feat: initialize letters app and register it in settings

backend/letters/admin.py

@@ -0,0 +1 @@
+# Register your models here.

backend/letters/apps.py

@@ -0,0 +1,5 @@
+from django.apps import AppConfig
+
+
+class LettersConfig(AppConfig):
+    name = "letters"

backend/letters/models.py

@@ -0,0 +1 @@
+# Create your models here.

backend/letters/tests.py

@@ -0,0 +1 @@
+# Create your tests here.

backend/letters/views.py

@@ -0,0 +1 @@
+# Create your views here.