Prepare free BorderPulse app for a cost-controlled Vercel preview

[sbc1-code]

Purpose

Prepare the current free, ad-supported BorderPulse app for a Vercel preview without switching the public domain or introducing Supabase, Sentry, auth, or billing.

Changes

• Serve the same normalized, identity-pinned official CBP data from a five-minute CDN-cached public function on Vercel.
• Use the existing static snapshot if that function fails, preserving the snapshot's actual timestamp and the last successful in-browser reading.
• Materialize compact 30-day history in the scheduled data writer so Vercel and Pages builds no longer need Git history.
• Keep GitHub Pages on its existing static path and preserve the current automatic Adsterra UI. No new dependencies.

Cost boundary

Do not connect each 15-minute data commit to a Vercel frontend build. The live function fetches data independently; the history artifact is refreshed by the existing GitHub writer. A production move still needs a preview readback, usage/spend review, an update plan for historical aggregates, and Sebastian's explicit approval.

Review evidence

• git diff --cached --check passed before commit; snapshot writer produced 388 compact observations (820 KB) from the current 30-day history.
• GitHub release checks are pending at PR creation. No local test suite was run.
• No production deployment or domain change in this PR.

Tracks #101.