docs: add frontend integration guide

[trenchsheikh]

Adds backend/docs/FRONTEND_INTEGRATION.md, a practical guide for the
frontend team on how to wire up to the UnderWriter backend.

What's covered

• Servers and ports, plus the recommended Next.js rewrite to avoid CORS.
• All four endpoints (/api/health, /api/run, /api/memo/{runId}, /api/amend) with request/response shapes.
• The shared contract.ts types (DeskFinding, Verdict, RunEvent, MemoData, OverrideContext, AmendmentDraft) and three options for sharing the file with the frontend.
• The SSE run flow: guaranteed event order, why EventSource won't work for POST, a copy-pasteable streaming reader (lifted from scripts/smoke.ts), and a suggested useReducer state model.
• End-to-end UI choreography for the two seeded demos (clean-acme PASS / bec-acme BLOCK + amendment).
• Error-handling tiers, frontend env vars, and a checklist for the UI build.

The guide is read-only documentation — no code changes — so no testing needed beyond rendering the markdown.

Slack Thread