Add liquidation imminent alerts

[Sikkra]

Closes #59

Summary

• add a separate LIQUIDATION IMMINENT email template for health-factor emergencies
• compute theoretical health factor per pool/asset/leverage bracket from live reserve collateral and liability factors
• send liquidation-imminent alerts independently of negative-APY alerts when HF is below 1.05
• rate-limit liquidation alerts separately per subscription to at most once per 6 hours
• optionally POST a liquidation webhook payload via LIQUIDATION_WEBHOOK_URL in parallel with email delivery
• add the D1 column/index and a defensive runtime column migration for existing deployments
• make Worker type-checking pass by adding Cloudflare worker types and removing a stale unused XDR helper

Verification

• npm ci in alerts
• npx tsc --noEmit in alerts
• npm test --if-present in alerts
• git diff --check