Add Slack and Discord alert webhooks

[Sikkra]

Summary

• add email/slack/discord alert channels with a unified notify() delivery path
• store channel + destination on subscriptions, including runtime schema upgrade/backfill for existing email-only D1 tables
• send webhook verification/test messages before saving Slack or Discord subscriptions, while preserving email verification links
• make the alerts package typecheckable by adding workers types, allowing .ts imports, and removing a stale undefined XDR helper

Closes #63.

Verification

• npm ci in alerts/
• npx tsc --noEmit -p .\tsconfig.json in alerts/
• mocked notify() smoke test for Slack verification, Discord verification, and Slack APY alert payloads
• Python sqlite in-memory execution of alerts/src/schema.sql plus unique-constraint check
• git diff --check