✨(backend) add 'sandbox' configuration class for demo environment

Create dedicated config for sandbox environment used for external demos,
bug bounties, and security auditing purposes.

src/backend/meet/settings.py

@@ -802,6 +802,14 @@ class Staging(Production):
     """
 
 
+class Sandbox(Production):
+    """
+    Sandbox environment settings
+
+    nota bene: it should inherit from the Production environment.
+    """
+
+
 class PreProduction(Production):
     """
     Pre-production environment settings