Add a Discord notifier for job alerts

[peopleworks]

What

Add a Discord notifier so job alerts can post to a Discord channel, alongside the existing email and webhook notifiers.

Why

Many teams live in Discord. A native notifier (or a thin adapter over the existing webhook) makes alerts land where people already are.

Suggested approach

• Mirror the existing webhook/email notifier shape.
• Accept a Discord webhook URL from configuration (treat it as a secret — never log it).
• Format the alert as a Discord embed (title, status color, job name, exit code, short output tail).

Acceptance criteria

• New notifier configurable via appsettings.json
• Fires on the same conditions as email/webhook alerts
• Webhook URL is never written to logs
• A short docs note in the README configuration section

Good first issue — say hi in the comments and it's yours. 🙌