Deploy with production public env; roll back unready releases

Imported from GitHub pull request El-SaMa/hollahoop#111 by @El-SaMa.

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Incident 2026-10-23 ~17:25-18:45 UTC: the first automatic deploys shipped a build made with NEXT_PUBLIC_SUPABASE_URL=https://example.supabase.co (CI placeholder). Liveness passed, so no rollback; every page and API call failed with fetch failed. Rolled back by hand to 20261014T200359.

• deploy.yml builds with vars.NEXT_PUBLIC_* from the production environment (set), fails when missing.
• New /api/v1/health/ready: 503 on placeholder or unreachable Supabase (/auth/v1/health), with tests.
• deploy.sh checks ready after liveness and rolls back on failure.

<!-- hh:closed-by:https://github.com/El-SaMa/hollahoop/pull/111 -->

Closed by Deploy with production public env; roll back unready releases