Serve from Firebase Hosting and redirect the Vercel address #132

What to build

This slice moves the app to Firebase Hosting on the *.web.app address, following ADR 0001 in #122. PR previews come from Firebase preview channels instead of Vercel.

After the Neon copy (#131) has been checked, the old Vercel address keeps serving the read-only old app from #130 for six months instead of redirecting. An installed copy that was offline through the switch can then still update, save its unsent changes to a file, and follow a link to the new address. Only after that period is the Vercel project reduced to a redirect.

The redirect lands on a short note saying installed copies of the app need to be installed again from the new address.

This is HITL because it needs access to the Firebase and Vercel project settings and the timing of the switch.

Acceptance criteria

• The app deploys to Firebase Hosting from CI on merge to main
• PRs get a Firebase preview channel link
• The PWA works from the new address, including offline
• After One-time copy from Neon to Firestore #131 is checked, the old address serves the read-only app with a prominent link to the new address
• After six months, every path on the Vercel address redirects to the same path on the new address
• Visitors arriving through the redirect see a one-time note about reinstalling the app
• The dates of both steps are recorded in the runbook

Blocked by

• One-time copy from Neon to Firestore #131