feat(12-02): add refresh_cloud_folder Celery task, staleness trigger, version 0.1.5

- Add tasks/cloud_tasks.py: durable refresh_cloud_folder task with 3-retry
  bounded backoff (30s/90s/270s +jitter), credential decryption in worker only
- Register tasks.cloud_tasks.* on documents queue in celery_app.py
- Add stale-while-revalidate staleness trigger in browse.py (5-min threshold)
- Add 4 Task 3 tests: idempotency, cached-row retention on failure, task structure,
  no-byte-download contract; add background-refresh scheduling integration test
- Bump backend version 0.1.4 → 0.1.5, frontend package.json 0.1.4 → 0.1.5
- Update AGENTS.md with Phase 12 Plan 02 state and new shared module map entries
- Update README with connection-ID browse API table and v0.1.5

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "document-scanner-frontend",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "type": "module",
   "scripts": {
     "dev": "vite",