[S7] Add Prometheus metrics

[krakenhavoc]

Description

Export metrics for monitoring the GitHub integration.

Metrics

• krakenkey_github_webhook_received_total (counter, by event_type)
• krakenkey_github_webhook_processing_duration_seconds (histogram)
• krakenkey_github_reconciliation_total (counter, by trigger, status)
• krakenkey_github_reconciliation_duration_seconds (histogram)
• krakenkey_github_cert_operation_total (counter, by operation: request/renew, status)
• krakenkey_github_deploy_total (counter, by target_type, status)
• krakenkey_github_installations_active (gauge)
• krakenkey_github_queue_depth (gauge, by queue)
• krakenkey_github_rate_limit_remaining (gauge, by installation)

Acceptance Criteria

• All metrics exported to /metrics endpoint
• Metrics are accurate under load
• Dashboard-ready (include example Grafana dashboard JSON if time permits)