Add health factor alert thresholds

[Sikkra]

Summary

• Adds hf_threshold and last_hf_alerted_at subscription fields plus an HF due index.
• Extends subscription validation/storage and the alert modal so users submit an HF threshold with their APY alert.
• Computes reserve health factor per leverage bracket in the cron path and sends a separate throttled HF breach email when the estimated HF is at or below the subscriber threshold.
• Adds the missing Cloudflare workers type dependency and tsconfig option needed for the alerts worker typecheck.

Verification

• npm run build from frontend/
• npm ci; npx tsc --noEmit from alerts/
• git diff --check
• Headless Edge/CDP check on http://127.0.0.1:5198/: alert modal showed the HF threshold field and submitted hf_threshold: 1.07 in the /subscribe payload; no console warnings/errors or runtime exceptions were reported.

Closes #58