Add /docs endpoint with OpenAPI schema

[bburda]

Summary

Implement a /docs endpoint that serves the OpenAPI schema for the gateway REST API. This enables API documentation discovery and integration with tools like Swagger UI.

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Proposed solution

1. Add a GET /api/v1/docs endpoint that returns the OpenAPI 3.x JSON/YAML schema
2. The schema should be auto-generated
3. Consider adding a /docs/uior /swagger-ui endpoint serving Swagger UI for interactive documentation
4. Schema should reflect actual implemented endpoints and their request/response models

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Additional context

• Consider caching the schema to avoid regeneration on each request