feat: refresh feeds hourly and repurpose header refresh

[phatpham9]

Summary

Update feedreader to refresh upstream sources hourly by default and keep the header refresh button as a lightweight backend re-fetch control.

What changed

• change the default scheduler cadence from every 3 hours to every 1 hour
• make the scheduler honor FEEDREADER_REFRESH_INTERVAL_HOURS when calculating the next wall-clock refresh boundary
• keep the header refresh button, but repurpose it to re-fetch the current feed view from /api/items
• do not use the header button to trigger upstream source refreshes
• keep on-demand upstream source refresh available through the CLI (feedreader fetch) and POST /api/refresh
• update the README to distinguish backend view refresh from upstream source refresh
• refresh the checked-in README screenshot asset to show the restored header button
• bump the app JS query-string version and service-worker shell cache key so deployed clients do not keep stale cached refresh behavior

Verification

• docker run --rm -v "$PWD":/src -w /src golang:1.24-bookworm go test ./...
• verified in a live local preview on http://127.0.0.1:18082
• confirmed in browser automation that clicking the header refresh button issued:
  • GET /api/items?...
  • 0 requests to /api/refresh
• confirmed the README screenshot asset was regenerated from the updated working-tree preview

Notes

• the scheduler remains aligned to Asia/Ho_Chi_Minh / UTC+7 wall-clock boundaries
• the default interval is now 1 hour, but FEEDREADER_REFRESH_INTERVAL_HOURS still overrides it