streamdata: edit metadata

This edits metadata in a robust way.

Signed-off-by: Yonah <contrib@gensokyo.uk>

streamdata.go

@@ -130,6 +130,8 @@ type VOD struct {
 	Date time.Time `json:"date"`
 	// Free-form category string.
 	Category string `json:"category,omitempty"`
+	// A mirror site the asset is uploaded to, usually YouTube.
+	Mirror string `json:"mirror,omitempty"`
 }
 
 // ChannelMismatchError describes a mismatching [Ident.Channel] passed to [Channel.Add].