Add a photo or video to a slideshow

Adds a photo or video as a slide, shown exactly as it is; nothing checks what the picture says. It is also the way to add a slide the layouts of POST /v1/slideshows/{id}/slides/design cannot make: design the slide as an image and add it here. Send exactly one of mediaUrl (downloaded and stored) or mediaAssetId (from POST /v1/media), plus optional durationSeconds and position (zero-based; appended when omitted): {"mediaUrl": "https://example.com/poster.jpg", "durationSeconds": 10, "position": 0}. A video plays its own length. An aspect-ratio mismatch attaches an AspectMismatch warning rather than failing. Queues a re-render. To have the picture turned into a designed slide instead, use POST /v1/slideshows/{id}/slides/ai.