Bump API pin to 3.0.0-rc6

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: turegjorup

.env.example

@@ -27,7 +27,7 @@ OS2DISPLAY_ADMIN_CLIENT_PATH=/admin
 
 # ghcr.io/os2display/display-api-service tag (api + nginx, both share this version).
 # https://github.com/os2display/display-api-service/pkgs/container/display-api-service
-OS2DISPLAY_VERSION_API=3.0.0-rc5
+OS2DISPLAY_VERSION_API=3.0.0-rc6
 
 # Engine name of the docker network that fronts the stack. Default "frontend".
 # Override + include compose.shared-frontend.yml via COMPOSE_FILE if running