chore: report cron week lookup errors clearly

[cursor]

Summary

• Check the current_week RPC error in the demo-day cron route before using its data
• Return a clearer 500 response that includes the Supabase error message when the week lookup fails

Why it matters

This makes cron failures easier to diagnose instead of reporting every week lookup problem as a generic missing-week result.

Verification

• npx eslint src/app/api/cron/demo-day/route.ts
• npx tsc --noEmit --pretty false --target ES2017 --lib dom,dom.iterable,esnext --module esnext --moduleResolution bundler --jsx react-jsx --strict --esModuleInterop --skipLibCheck src/app/api/cron/demo-day/route.ts

Note: npm run build compiled successfully, then failed during global type checking because open-next.config.ts imports @opennextjs/cloudflare, which is not installed from the current package.json in this environment.