feat: add slack outage notifications

[vahapogut]

Fixes #23

Summary

• Add SLACK_WEBHOOK_URL support alongside the existing Discord webhook.
• Send outage and resolution notifications to every configured webhook channel.
• Pass the Slack webhook secret through the scheduled status workflow and update the local test helper.

Verification

• bun run format:check
• bun x tsc --noEmit
• bun run scripts/discord-notification.ts exits with code 1 when no webhook env vars are configured
• git diff --check

[vercel]

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