fix: feature gate some guest features in the backend

crates/chattyness-db/src/models.rs

@@ -451,6 +451,13 @@ pub struct User {
     pub updated_at: DateTime<Utc>,
 }
 
+impl User {
+    /// Check if this user is a guest (has the Guest tag).
+    pub fn is_guest(&self) -> bool {
+        self.tags.contains(&UserTag::Guest)
+    }
+}
+
 /// Minimal user info for display purposes.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct UserSummary {