[US-030] OpenAPI spec auto-generated from Django REST Framework

[leimo2011]

User Story

As a developer,
I want an always-up-to-date OpenAPI spec auto-generated from the codebase,
So that documentation never drifts from the actual API.

Acceptance Criteria

• drf-spectacular configured to generate OpenAPI 3.0 spec
• Swagger UI accessible at /api/docs/
• Redoc accessible at /api/redoc/
• All endpoints documented with request/response schemas
• Spec exported as openapi.yaml in liemo-docs repo on CI

Estimate: 3 SP | Milestone: M1

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Epic: [EPIC-16] API Documentation & Developer Experience