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

backend/celery_app.py

@@ -35,6 +35,7 @@ celery_app.conf.task_routes = {
     "tasks.document_tasks.*": {"queue": "documents"},
     "tasks.email_tasks.*": {"queue": "email"},
     "tasks.audit_tasks.*": {"queue": "documents"},
+    "tasks.cloud_tasks.*": {"queue": "documents"},
 }
 
 # Celery beat schedule:
@@ -55,5 +56,6 @@ celery_app.conf.timezone = "UTC"
 # Explicitly import task modules — autodiscover_tasks(["tasks"]) looks for
 # tasks.tasks (appends ".tasks") which doesn't exist in our structure.
 import tasks.audit_tasks  # noqa: F401, E402
+import tasks.cloud_tasks  # noqa: F401, E402
 import tasks.document_tasks  # noqa: F401, E402
 import tasks.email_tasks  # noqa: F401, E402