Generate OpenAPI/Swagger API documentation

[trsdn]

Summary

Add API documentation using OpenAPI specification for better developer experience.

Tasks

• Create openapi.yaml or openapi.json specification
• Document all API endpoints with request/response schemas
• Add Swagger UI endpoint (/api/docs)
• Add request/response examples
• Generate TypeScript types from OpenAPI spec (optional)
• Add API versioning header

Acceptance Criteria

• OpenAPI 3.0 spec covers all endpoints
• Swagger UI accessible at /api/docs
• All request/response schemas documented
• Examples provided for each endpoint