Document media-server collection create contract drift

[Copilot]

Maintainerr’s next release adds an optional initialItemId field to CreateCollectionParams so Emby collection creation can be seeded with one item. The docs/OpenAPI spec did not expose that request contract for POST /api/media-server/collection.

• What was added
  • Updated docs/API.md to describe the CreateCollectionParams body for POST /api/media-server/collection, including optional initialItemId.
  • Added CreateCollectionParams to the bundled OpenAPI spec and wired it to the create-collection endpoint.
  • Source: fix(emby): create collections with an initial item to avoid HTTP 500 Maintainerr#3097 / 179215b2.

CreateCollectionParams:
  required:
    - libraryId
    - title
    - type
  properties:
    initialItemId:
      type: string

• Already covered by prior docs
  • Plex watch-history empty-vs-failed behavior is already covered in Rules/Glossary docs.
  • Download-client, overlay, and collection terminology drift were already covered by recent merged docs PRs.