fix: chat accepts posts

internal/model/chat.go

@@ -173,6 +173,11 @@ const (
 	AttachmentQuote = "quote"
 )
 
+// Entity types a message can reference.
+const (
+	EntityPost = "post"
+)
+
 // Attachment is a media file, link preview or shared entity on a message.
 //
 // Media carries an R2 key rather than a URL: chat media lives in the private
@@ -201,6 +206,16 @@ type Attachment struct {
 	SiteName    *string `json:"site_name,omitempty"`
 	ImageURL    *string `json:"image_url,omitempty"`
 
+	// Shared entity (a post, for now). EntityType/EntityID are the durable
+	// reference; Entity is resolved per viewer at read time, so the same row
+	// renders differently — or not at all — depending on who is looking.
+	EntityType *string         `json:"entity_type,omitempty"`
+	EntityID   *string         `json:"entity_id,omitempty"`
+	Entity     json.RawMessage `json:"entity,omitempty"`
+	// EntityUnavailable is set when the entity was deleted, hidden, or the
+	// viewer has since lost access. The client renders a tombstone.
+	EntityUnavailable bool `json:"entity_unavailable,omitempty"`
+
 	CreatedAt time.Time `json:"created_at"`
 }