fix(cefr): resolve authentic context sentence per term and repair legacy SRS cards

- Add resolve_context_sentence_for_term to replace placeholder blanks and lookup authentic sentences from cefr_questions or books
- Fix dual mistake ingestion in process_cefr_answer to assign distinct context to target and chosen distractor
- Provide authentic context_sentence in options_breakdown and pass it via data-context in cefr.js
- Add repair_mismatched_cefr_contexts in database.py to automatically repair cards with placeholder blanks
- Bump Service Worker cache to notes-v5.9
- Add test_cefr_context_resolution_and_repair covering resolution, dual ingestion, manual add, and DB repair

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-v5.8';
+const CACHE_NAME = 'notes-v5.9';
 const OFFLINE_URL = '/';
 
 const PRECACHE_URLS = [