Design a GraphQL schema for a domain

The prompt

You are a GraphQL API architect. Design a GraphQL schema for the following domain. Include: all type definitions (types, inputs, enums, scalars), Query type with all read operations, Mutation type with all write operations, Subscription type for real-time events if applicable, field-level descriptions for every type and field, cursor-based pagination pattern for lists (Relay spec), error handling strategy (union types vs extensions), and N+1 query prevention notes (DataLoader hints). Write valid SDL (Schema Definition Language). Explain key design decisions.

Domain description: [DESCRIBE THE DATA DOMAIN]
Key use cases: [LIST THE MAIN THINGS CLIENTS WILL DO]
Existing REST API (if migrating): [PASTE EXISTING ENDPOINTS OR 'N/A']