feat: Replace sync normalized-data with async flow (API-432)

[hilnius]

Summary

• Replace the sync POST /generate-normalized-data call with the async submit + poll pattern (POST /generate-normalized-data-async + GET /generate-normalized-data-async/{id}).
• Add a reusable pollUntilSucceeded helper in frontend/src/api/async-job.ts (10 s poll interval, uppercase job statuses).
• Update the full-encounter demo docs badge and documentation link to point at the async endpoint.

Stacked on #45 (API-431) — merge that PR first.

Linear: https://linear.app/nabla/issue/API-432/sample-app-replace-sync-normalized-data-with-async-flow

Test plan

• Full Encounter Demo → generate note → Extract normalized data completes successfully
• Network tab shows POST .../generate-normalized-data-async followed by GET .../generate-normalized-data-async/{id} polls (~10 s apart)
• Failed jobs (status: FAILED) stop polling and surface an error
• Doc link opens the async endpoint documentation page

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit d3533d1. Configure here.}