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/api/cloud/browse.py

@@ -245,11 +245,18 @@ async def browse_connection_items(
             for action in ACTIONS
         }
 
-    # Refresh in background if first visit (no items) or stale
+    # Refresh in background if first visit (no items), currently refreshing,
+    # never refreshed, or stale (last refresh > 5 minutes ago)
+    import datetime as _dt
+    _stale_threshold = _dt.timedelta(minutes=5)
+    _is_stale = (
+        folder_state.last_refreshed_at is None
+        or (_dt.datetime.now(_dt.timezone.utc) - folder_state.last_refreshed_at) > _stale_threshold
+    )
     should_refresh = (
         not cached_items
         or folder_state.refresh_state in ("refreshing",)
-        or folder_state.last_refreshed_at is None
+        or _is_stale
     )
     if should_refresh and not cached_items:
         # First visit: do a synchronous bounded fetch so user sees content