v4.2: Improve API

[thorsten]

phpMyFAQ needs a consistent, documented Read API as a foundation for headless frontends, integrations, and future AI features.

Scope

• Consistent response structures

Acceptance Criteria

• A valid OpenAPI specification exists
• All read endpoints return consistent JSON responses
• Pagination, sorting, and filtering are standardized
• Errors follow RFC 7807 (application/problem+json)
• Endpoints are covered by automated tests