feat: configure Kanban login with optional email

docker-compose.yml

@@ -12,7 +12,10 @@ x-app-environment: &app-environment
   AWS_ACCESS_KEY_ID: ${R2_ACCESS_KEY_ID:?set R2_ACCESS_KEY_ID}
   AWS_SECRET_ACCESS_KEY: ${R2_SECRET_ACCESS_KEY:?set R2_SECRET_ACCESS_KEY}
   AWS_DEFAULT_REGION: auto
-  OPERATOR_USER: operator
+  # Optional at deploy time so a missing Kanban credential cannot make the
+  # entire Swarm stack invalid. The Kanban login endpoint fails closed until
+  # this value is configured.
+  OPERATOR_EMAIL: ${OPERATOR_EMAIL:-}
   OPERATOR_PASSWORD: ${OPERATOR_PASSWORD:?set OPERATOR_PASSWORD}
   PAYMENT_ADAPTER: fake
   FREIGHT_ADAPTER: fake