video thumbnail generation

still not perfect

actually cache thumbnail
simplify media id

AobaServer/Utils/ProtoExtensions.cs

@@ -44,15 +44,18 @@ public static class ProtoExtensions
 
 	public static MediaModel ToMediaModel(this Media media)
 	{
+		var thumbUrl = $"/m/{media.MediaId}/thumb?size={ThumbnailSize.Medium}";
+		if (media.Thumbnails.TryGetValue(ThumbnailSize.Medium, out var thumb))
+			thumbUrl = $"/t/{thumb}";
 		return new MediaModel()
 		{
 			Ext = media.Ext,
 			FileName = media.Filename,
-			Id = media.Id.ToId(),
-			MediaId = media.MediaId.ToId(),
+			Id = media.MediaId.ToId(),
 			MediaType = (Aoba.RPC.MediaType)media.MediaType,
 			Owner = media.Owner.ToId(),
 			ViewCount = media.ViewCount,
+			ThumbUrl = thumbUrl,
 		};
 	}