Migrate a 60-endpoint Express REST API to GraphQL without breaking mobile clients

Our logistics tracking product has grown a REST API of about 60 endpoints over four years, and our mobile team now spends more time stitching responses together than building features. We want a GraphQL layer introduced incrementally - schema-first, resolvers backed by the existing PostgreSQL database, with the old REST routes kept alive until every client has moved over. Deliverables: an agreed schema design document, the GraphQL server itself (Node.js, TypeScript, Apollo or Yoga - your call, argued for in writing), dataloader-based batching to kill the N+1 queries we know are coming, and a deprecation plan for the REST endpoints. Test coverage on resolvers is non-negotiable; we run CI on every PR and nothing merges red. You would work directly with our two backend engineers in a shared repo. We are in Central European time and prefer contractors who overlap at least three hours with Berlin business hours.