feat: order streams by last child creation

js/src/api/types.ts

@@ -138,6 +138,9 @@ export const ParticleSchema = z.discriminatedUnion("type", [
     visible_to: z.array(z.string()),
     // Marks emails to their `playback_position_at`: where they left off in a conversation
     markers: z.record(z.string(), z.coerce.date()).optional(),
+    // Timestamp of the most recent child particle
+    //   used for sorting streams by recent activity without needing to query subcollections
+    last_child_created_at: z.coerce.date().optional(),
   }),
   ParticleBaseSchema.extend({
     type: z.literal("folder"), properties: FolderPropertiesSchema,