backend-contract-boundarylisted

# Skill: Backend Contract Boundary ## Intent Ensure backend requirements produce stable, testable contracts instead of ad-hoc implementation code. This skill protects API boundaries, domain boundaries, persistence boundaries, adapter boundaries, and error semantics. ## Use when Use this skill when a REQ touches backend services, APIs, controllers, routes, workers, domain logic, persistence, queues, webhooks, adapters, SDK integrations, authentication, authorization, or service contracts. ## Do not use when Do not use this skill for purely frontend visual work, static documentation, or isolated scripts with no public or internal contract. ## Signals - The REQ mentions API, endpoint, service, route, FastAPI, Flask, Django, Express, NestJS, worker, queue, database, repository, adapter, webhook, schema, DTO, OpenAPI, contract, persistence, auth, or domain boundary. - Generated files include backend source, migrations, service classes, adapters, routers, validators, or integration tests. - Acceptance criteria depend on request/response behavior, events, persistence effects, or external integration semantics. ## Required behavior - Keep public contracts explicit: request schema, response schema, event payload, command shape, or adapter interface. - Validate inputs at the boundary. - Keep domain logic independent from transport details when practical. - Keep external systems behind adapters or ports. - Preserve backward compatibility unless the REQ explicitly allows a bre