feat: add stickers

internal/model/chat.go

@@ -171,11 +171,16 @@ const (
 	// app-relative deep link back to the phrase, title the source reference,
 	// description the passage itself.
 	AttachmentQuote = "quote"
+	// AttachmentSticker is reward artwork: image_url and title carry the art
+	// and its name, entity_id the reward key a recipient can save by.
+	AttachmentSticker = "sticker"
 )
 
 // Entity types a message can reference.
 const (
 	EntityPost = "post"
+	// EntitySticker marks a sticker attachment's entity_id as a reward key.
+	EntitySticker = "sticker"
 )
 
 // Attachment is a media file, link preview or shared entity on a message.
@@ -257,6 +262,17 @@ type Message struct {
 	Reactions []Reaction `json:"reactions"`
 }
 
+// Sticker is one piece of reward artwork, as it appears in the picker and on
+// a message.
+type Sticker struct {
+	Key      string `json:"key"`
+	Name     string `json:"name"`
+	ImageURL string `json:"image_url"`
+	// Saved is true when it came from a chat rather than from sādhana, so the
+	// picker can say where it came from.
+	Saved bool `json:"saved"`
+}
+
 // ReactionDetail is one person's reaction, for the "who reacted" list.
 type ReactionDetail struct {
 	User      ChatPeer  `json:"user"`