feat: background tasks manager and book sentence mining for CEFR tests

app/static/sw.js

@@ -1,7 +1,7 @@
 // Service Worker for Notes English PWA (Stage 2)
 // Strategy: Cache-first for static assets, Network-first for API calls.
 
-const CACHE_NAME = 'notes-v5.4';
+const CACHE_NAME = 'notes-v5.5';
 const OFFLINE_URL = '/';
 
 const PRECACHE_URLS = [