feat(monitoring): add failure alerting and escalation chain for cron job failures

[Darkdruce]

Closes #759

Adds persistent cron job failure tracking with escalating alerts.

• New Supabase migration 014_cron_job_failures.sql creates cron_job_failures table (survives restarts)
• CronFailureTrackerService with recordSuccess, recordFailure, and wrapCronHandler
• Alert thresholds: Slack webhook at 3 consecutive failures, email (console error) at 6
• Auto-resolves on successful run (resets counter to 0)
• Secrets: SLACK_WEBHOOK_URL env var; no alert sent if unset
• Tests cover failure counting, escalation triggers, auto-resolve, and handler wrapping

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