Per-branch Vercel preview deployments for DPROD

[jgeluk]

Goal

Host DPROD on Vercel so that every branch and PR automatically gets its own preview URL, instead of the current situation where only `main` is published (as GitHub Pages at https://ekgf.github.io/dprod, showing the OMG-approved 1.0). The `develop` branch and in-flight PR branches should all be reachable at stable URLs for review.

Desired behavior

• `develop` is the production branch on Vercel (not `main`), so the canonical site reflects ongoing work.
• `main` stays pinned as the historical OMG 1.0 via a stable alias (e.g. `1.0.dprod.ekgf.org` or similar).
• Every branch push produces a preview like `dprod-git--ekgf.vercel.app`.
• Every PR gets its preview URL posted back as a comment automatically.
• Custom domain: `dprod.ekgf.org` (or equivalent) points at the Vercel project.

Depends on

• Marketing / explainer website for DPROD #174 (marketing site) — Vercel project will be created as part of or after that work, since the Next.js app is what Vercel builds.

Priority

Medium — lower than the marketing site itself, but it's the mechanism that makes in-flight ballot work reviewable.