AIRA-38: PagerDuty Webhook Handler

[learn-design-develop]

👤 User Story

As the core processing layer, I need a comprehensive webhook handler for processing PagerDuty incident events with proper validation and error handling.

🎯 Rationale

Webhook Handler is the critical entry point for incident notifications.

✅ Acceptance Criteria

• Create aira/core/webhook_handler.py with processing class
• Implement POST /webhooks/pagerduty endpoint
• Define Pydantic models for payloads (PagerDutyWebhookEvent, PagerDutyIncident)
• Implement HMAC-SHA256 signature validation using X-PagerDuty-Signature
• Use PAGERDUTY_WEBHOOK_SECRET config
• Map event types to internal format
• Request/response logging with correlation IDs (via shared utility)
• Add webhook processing metrics (success_count, error_count, processing_duration)
• Comprehensive unit & integration tests

📋 Metadata

• Status: MVP

• Category: Security

• Week: Week 3

• Complexity: Medium

• Critical Path: Yes

• Dependencies: AIRA-34, AIRA-31

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Original Ticket: #38
Phase 1 MVP Tracking Issue