refactor(lib): add usePolling hook for stats and admin

[pq198363-ops]

Summary

• Add a reusable usePolling<T> hook that uses the shared apiGet client, fetches immediately, polls on an interval, supports pause/resume/manual refresh, tracks lastUpdated, and aborts superseded or unmounted requests.
• Refactor the stats page to use usePolling<Stats>("/api/v1/stats", 5000) instead of a local setInterval loop.
• Refactor the admin status panel to use the same hook for /api/v1/admin/status, while still awaiting an immediate refresh after pause/unpause actions.
• Document the hook contract and current adopters in docs/hooks.md and README.md.
• Restore existing missing imports for Breadcrumb, formatRequests, and safeHref so the repository build/test pipeline can complete on this PR.

Closes #73

Validation

• npx jest src/lib/__tests__/usePolling.test.tsx src/app/stats/page.test.tsx src/app/admin/page.test.tsx src/app/admin/__tests__/admin-page-stale-guard.test.tsx --runInBand
  • 4 suites passed; 23 tests passed; 2 skipped existing skipped tests.
• npx jest src/lib/__tests__/usePolling.test.tsx src/app/stats/page.test.tsx src/app/admin/page.test.tsx src/app/admin/__tests__/admin-page-stale-guard.test.tsx --coverage --collectCoverageFrom=src/lib/usePolling.ts --collectCoverageFrom=src/app/stats/page.tsx --collectCoverageFrom=src/app/admin/page.tsx --coverageReporters=text --coverageThreshold='{}' --runInBand
  • src/lib/usePolling.ts: 100% statements, 100% branches, 100% functions, 100% lines.
  • src/app/stats/page.tsx: 100% statements, 100% branches, 100% functions, 100% lines.
  • src/app/admin/page.tsx: 100% statements, 97.82% branches, 100% functions, 100% lines.
• npx eslint src/lib/usePolling.ts src/lib/__tests__/usePolling.test.tsx src/app/stats/page.tsx src/app/stats/page.test.tsx src/app/admin/page.tsx src/app/admin/page.test.tsx src/app/admin/__tests__/admin-page-stale-guard.test.tsx
• npx tsc --noEmit --pretty false --skipLibCheck --lib dom,dom.iterable,esnext src/lib/usePolling.ts
• git diff --cached --check
• npm run typecheck
• npm run lint
  • Passes with existing warnings only in src/app/webhooks/page.tsx for unused EmptyState and Spinner imports.
• npm run build
• npm test -- --runInBand
  • 62 suites passed; 606 tests passed; 2 skipped.

Notes

The second commit only restores imports for existing symbols that were already referenced on main; it is included here because the PR build runs the full frontend pipeline before reaching the focused hook tests.