Add Convex prod deployment to CI/CD pipeline

[SebastienMelki]

Context

Convex production deployment (npx convex deploy) is currently manual. After merging to main, someone has to remember to run it locally. This is error-prone — Vercel deploys automatically but Convex doesn't, so the frontend and backend can drift.

What

Add a GitHub Actions workflow that runs npx convex deploy on push to main.

Requirements

• Trigger on push to main (or on PR merge)
• Run npx convex deploy --cmd 'npm run build' with CONVEX_DEPLOY_KEY secret
• Only deploy if convex/ files changed (optional optimization)
• Should run after tests pass

References

• Convex CI/CD docs: https://docs.convex.dev/production/hosting/
• Current prod deployment: tacit-curlew-777
• Dev deployment: valiant-bison-406

Came up during the better-auth integration (PR #1812) — adding auth plugins requires Convex redeployment which currently isn't automated.