[M1][Epic 1.5] Implement threshold-based alerting with webhook notifications

[POWDER-RANGER]

Problem

Start with single alert rule type (average sentiment threshold) and single notification channel (webhook or console log for MVP).

Tasks

• Create alert rule schema (threshold type: avgSentiment > threshold)
• Implement alert evaluation logic after ingestion completes
• Store triggered alerts in alerts table
• Choose notification channel (webhook recommended, email acceptable)
• Implement notification sender
• Add alert polling or WebSocket for frontend real-time updates
• Create GET /api/alerts/recent endpoint

Acceptance Criteria

• ✅ Alert created when avgSentiment crosses threshold
• ✅ Alert appears in dashboard within 5 seconds of ingestion
• ✅ Notification sent to configured channel
• ✅ Alert history queryable via API

Priority: P1 - High
Labels: backend, alerts, M1, P1