Designs scalable backend architectures, models relational database schemas, builds REST/GraphQL/gRPC APIs, configures cloud infrastructure, and implements microservices with security and observability built in. Use when asked to design a backend system, create or version an API, write a database schema or migration, set up microservices, plan cloud deployment architecture, implement authentication/authorization, configure caching or message queues, or optimize server-side performance. Covers PostgreSQL, Redis, RabbitMQ, Docker, Kubernetes, and Infrastructure as Code.