Instruction file imported from vokerg/chess_repertoir_trainer (.github/instructions/contracts.instructions.md). Copyright stays with the author.

Contract changes

• Inspect the real API route, mapper/service output, Angular consumer, and tests before defining a schema.
• Store only HTTP wire shapes and stable wire literals here. Do not add Prisma, Fastify, UI state, chess behavior, or workflows.
• Export each Zod schema and its inferred input/output type.
• HTTP dates are ISO strings. Model null with .nullable() and absence with .optional() only when each is actually part of the contract.
• Test representative real mapper/service output and rejected invalid output.
• Migrate the API route schema, generated OpenAPI, and Angular type import in the same endpoint slice; then delete the duplicate DTO/schema.
• Validate contracts, API, and web builds for shared-contract changes.