feat: add event log filters

[gaoflow]

Summary

• add event-type and payload substring filters to the event log page
• show Showing X of Y events so active filters are clear
• use the shared EmptyState for no events and no matching results
• add tests for source ordering, type filter, payload filter, empty match, and empty event list

Closes #5

Verification

$ npm run lint

> stableroute-frontend@0.1.0 lint
> next lint

✔ No ESLint warnings or errors

$ npm test -- --runInBand

> stableroute-frontend@0.1.0 test
> jest --runInBand

PASS src/app/events/page.test.tsx
PASS src/app/quote/page.test.tsx
PASS src/components/__tests__/Header.test.tsx
PASS src/__tests__/page.test.tsx
PASS src/components/__tests__/Card.test.tsx
PASS src/components/__tests__/Spinner.test.tsx
PASS src/components/__tests__/Badge.test.tsx
PASS src/lib/__tests__/format.test.ts

Test Suites: 8 passed, 8 total
Tests:       22 passed, 22 total
Snapshots:   0 total
Time:        0.808 s, estimated 1 s
Ran all test suites.

$ npm run build

> stableroute-frontend@0.1.0 build
> next build

✓ Compiled successfully
✓ Generating static pages (15/15)

Rendered smoke:

• Browser plugin was unavailable in this Codex session (Browser is not available: iab), so I used Playwright CLI fallback.
• npx playwright screenshot --full-page http://localhost:3000/events /tmp/stableroute-events-prod-smoke.png
• Production /events rendered the filter controls, count, and event rows without a framework overlay.

[mikewheeleer]

appreciate the PR @gaoflow 🙏 heads up though — #5 wasn't assigned to you, and we merge from the assigned contributor to keep it fair. please claim an unassigned issue first, then PR. closing for now 🙌