fix: add missing Video field to Embed model

The server returns a `video` object in embeds (e.g. for GIF providers)
but the Android model lacks the field, causing it to be silently
dropped during deserialization.

Signed-off-by: sanasol <mail@sanasol.ws>

core/model/src/main/java/chat/stoat/core/model/schemas/Messages.kt

@@ -70,11 +70,19 @@ data class Embed(
     val siteName: String? = null,
 
     val colour: String? = null,
+    val video: Video? = null,
     val width: Long? = null,
     val height: Long? = null,
     val size: String? = null
 )
 
+@Serializable
+data class Video(
+    val url: String? = null,
+    val width: Long? = null,
+    val height: Long? = null
+)
+
 @Serializable
 data class Image(
     val url: String? = null,