OpenAPI schema is broken

[khvn26]

Trying to generate an OpenAPI client from current schema results in errors:

This is due to drf-yasg usage, which does not support OAS 3.0. We need OAS 3.0 to properly express union types via anyOf.

In order to support OAS 3.0, we need to migrate our OpenAPI/Swagger generation to drf-spectacular.

• Replace drf-yasg dependency with drf-spectacular.
• Adapt api.openapi.PydanticResponseCapableSwaggerAutoSchema.
• Remove custom Meta and other means that produce the bespoke "type" union schema fields.
• Make sure the /api/v1/docs/?format=openapi provides the same output, but the union types are correctly expressed with anyOf.
• Make sure that the new schema is consumable by an OpenAPI code generator, e.g., @rtk-query/codegen-openapi.