Get rendered environment variables for a Railway service via the GraphQL API

Documented steps

1. Create a Railway API token (Account > Settings > Tokens) or use a workspace/project token; send it as the Authorization: Bearer header to the GraphQL endpoint https://backboard.railway.com/graphql/v2 with Content-Type: application/json
2. Run the query `query variables($projectId: String!, $environmentId: String!, $serviceId: String) { variables(projectId: $projectId, environmentId: $environmentId, serviceId: $serviceId) }` passing projectId, environmentId, and serviceId
3. Parse the response; the variables field returns an object mapping each variable name to its rendered (reference-resolved) value, e.g. {"DATABASE_URL":"postgres://...","NODE_ENV":"production"}
4. The token must have access to the project; workspace/project tokens scope data to their project