feat: add Django backend skeleton

docs/testing.md

@@ -8,6 +8,7 @@ All tests should run inside Docker containers.
 
 ```bash
 docker compose --env-file .env.example -f infra/docker/compose.yml config
+docker compose --env-file .env.example -f infra/docker/compose.yml run --rm --build backend python manage.py test
 ```
 
 ## Test categories
@@ -21,3 +22,8 @@ Describe applicable categories:
 - Ansible syntax checks;
 - Docker/Compose validation;
 - smoke tests.
+
+## Current coverage
+
+- Docker Compose configuration validation;
+- Django backend unit tests, including the initial health endpoint test.