getMedia method
Fetch a single media item by mediaId.
Throws CloudMediaNotFoundException if the item does not exist.
Implementation
Future<CloudMediaItem> getMedia(String mediaId) async {
final doc =
await _firestore.doc(FirestorePaths.mediaDoc(_uid, mediaId)).get();
if (!doc.exists) throw CloudMediaNotFoundException('Not found: $mediaId');
return CloudMediaItem.fromFirestore(doc);
}