Implements the monitoring alerting deliverable from Phase 6. The Alerter
module sends color-coded notifications to Discord and/or Slack webhooks
for operational events: bot health transitions, stale job re-enqueues,
and match errors. Includes per-key rate limiting to prevent alert storms.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
