Add daily summary email opt-in

[Sikkra]

Summary

• add daily summary opt-in controls to the alert subscription modal, including a user-selected UTC delivery hour
• persist summary preferences in D1 with runtime-safe migration columns for existing databases
• send compact daily digest emails with position rows, net APY, estimated HF, HF 24h delta, and estimated 24h yield
• add alerts typecheck support by declaring Cloudflare Worker types and enabling .ts imports

Closes #64

Verification

• npm run typecheck (alerts)
• npm run build (frontend)
• Python sqlite in-memory schema smoke for daily-summary columns and index
• Playwright/Edge smoke on local Vite port 5208: daily summary checkbox reveals UTC hour selector, subscribe payload includes daily_summary=true and summary_utc_hour=9, no browser console/page errors
• git diff --check