🔧(backend) activate container liveness probes

Enabled Dockerflow Django app by activating liveness probes. The previously
unavailable routes such as `__heartbeat__` and `__lbheartbeat__` are now
accessible. New endpoints include:
* GET /__version__
* GET /__heartbeat__
* GET /__lbheartbeat__

src/backend/impress/settings.py

@@ -177,6 +177,7 @@ class Base(Configuration):
         "django.middleware.csrf.CsrfViewMiddleware",
         "django.contrib.auth.middleware.AuthenticationMiddleware",
         "django.contrib.messages.middleware.MessageMiddleware",
+        "dockerflow.django.middleware.DockerflowMiddleware",
     ]
 
     AUTHENTICATION_BACKENDS = [