Improve Telegram alert messages

[nihey]

Summary

• Format immediate Telegram DOWN and recovery notifications as concise HTML status reports.
• Link each notification to the canonical organization/check page and include project, cadence, successful ping count, last ping, sibling health, and recovery downtime.
• HTML-escape user-controlled names, disable link previews, and leave email, Slack, and webhook formats unchanged.

Validation

• npm ci
• npx tsc -p tsconfig.build.json --noEmit
• npm test — 251 tests passed
• git diff --check

The test fixtures use generic public-safe names only.