♻️(backend) make SESSION_ENGINE configurable via environment variable

Make SESSION_ENGINE configurable through environment variable,
following the same values.Value() pattern already used for
SESSION_COOKIE_AGE. This enables OIDC backchannel-logout by
allowing users to set SESSION_ENGINE to db backend.

Closes #1037

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to
 
 ### Changed
 
+- ♻️(backend) configurable SESSION_ENGINE #1038 #1154
 - ♿️(frontend) fix sidepanel accessibility aria-label #1182
 - ♿️(frontend) fix more tools heading hierarchy #1181
 - ♿️(fronted) improve button descriptions for More tools actions #1184

src/backend/meet/settings.py

@@ -452,7 +452,11 @@ class Base(Configuration):
     CELERY_BROKER_TRANSPORT_OPTIONS = values.DictValue({}, environ_prefix=None)
 
     # Session
-    SESSION_ENGINE = "django.contrib.sessions.backends.cache"
+    SESSION_ENGINE = values.Value(
+        default="django.contrib.sessions.backends.cache",
+        environ_name="SESSION_ENGINE",
+        environ_prefix=None,
+    )
     SESSION_CACHE_ALIAS = "default"
     SESSION_COOKIE_AGE = values.PositiveIntegerValue(
         default=60 * 60 * 12, environ_name="SESSION_COOKIE_AGE", environ_prefix=None