🔧(back) configure cache key prefix

We want to change the cache key prefix using an environment variable.
This settings can be changed at every deployment in order to reset to
use a fresh new cache.

src/backend/impress/settings.py

@@ -913,6 +913,11 @@ class Production(Base):
             "OPTIONS": {
                 "CLIENT_CLASS": "django_redis.client.DefaultClient",
             },
+            "KEY_PREFIX": values.Value(
+                "docs",
+                environ_name="CACHES_KEY_PREFIX",
+                environ_prefix=None,
+            ),
         },
     }