fix(kiosk): allow X-Pair-Nonce through production CORS so kiosks can re-pair

[thewrz]

Why

A kiosk that gets unpaired from its event shows "Failed to create pairing session" and never displays a fresh QR code, so it can't be re-paired to another event.

The kiosk's POST /api/public/kiosk/pair carries a custom X-Pair-Nonce header, which triggers a browser CORS preflight. Production's non-wildcard CORS allow_headers never included X-Pair-Nonce, so the preflight returns 400 Disallowed CORS headers and the browser blocks the request before it's sent.

This has been latent since the nonce flow (#272): the live-display path uses X-Kiosk-Session (already allow-listed) so paired kiosks kept working, and local dev/tests use CORS_ORIGINS=* (allow_headers=["*"]) so it never reproduced. Unpairing forces the one code path the prod allow-list blocked.

What

• Add X-Pair-Nonce to the production CORS allow_headers in server/app/main.py.
• Add a regression test that builds the app under production-style explicit origins and asserts the kiosk pair-endpoint preflight allows x-pair-nonce (reproduced the exact 400 before the fix).

⚠️ Server-side fix — the kiosk only recovers after a backend redeploy to production.

Testing

• Backend unit tests pass (3062 passed; new test_cors.py regression test included)
• Coverage gate met (88.62% ≥ 85%)
• ruff check + ruff format clean; bandit clean
• Manual verification: after deploy, unpair the kiosk → it shows a fresh QR code and can pair to a new event
• CI green

🤖 Co-authored by Claude Opus 4.8. Closes #497.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 64e028c2-62a8-4c48-bbf8-f825dca910ec

📥 Commits

Reviewing files that changed from the base of the PR and between 002f752 and 54a1bdf.

📒 Files selected for processing (2)

• server/app/main.py
• server/tests/test_cors.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/kiosk-pair-cors-header

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