fix: add markAllRead function and route

Restore the "Mark all as read" button functionality in channel view.
The button form posts to /api/mark-read but the route and controller
function were missing from the standalone repo.

- Add markAllRead function to reader controller
- Add /api/mark-read route to index.js
- Import markItemsRead from storage/items.js

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

index.js

@@ -96,6 +96,7 @@ export default class MicrosubEndpoint {
     readerRouter.get("/search", readerController.searchPage);
     readerRouter.post("/search", readerController.searchFeeds);
     readerRouter.post("/subscribe", readerController.subscribe);
+    readerRouter.post("/api/mark-read", readerController.markAllRead);
     router.use("/reader", readerRouter);
 
     return router;