✨(backend) add env variable to configure marketing service timeout

This change allows the marketing service timeout to be easily adjusted
via an environment variable, eliminating the need for a new software release.
Additionally, the update makes the code more explicit and easier to maintain.

src/backend/core/authentication/backends.py

@@ -121,7 +121,9 @@ class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
             contact_data = ContactData(
                 email=email, attributes={"VISIO_SOURCE": ["SIGNIN"]}
             )
-            marketing_service.create_contact(contact_data, timeout=1)
+            marketing_service.create_contact(
+                contact_data, timeout=settings.BREVO_API_TIMEOUT
+            )
         except (ContactCreationError, ImproperlyConfigured, ImportError):
             pass