Documentation: Write API documentation

[copyleftdev]

Description

Create comprehensive API documentation.

Tasks

• Set up OpenAPI/Swagger
• Document all endpoints
• Add request/response examples
• Document error codes
• Add authentication guide
• Create getting started guide
• Add code examples in multiple languages

Documentation

• See docs/02-api-specification.md for complete spec

Acceptance Criteria

• OpenAPI spec is complete
• Interactive docs are available
• Examples are runnable