Phase 4: Add metrics, notifications, and monitoring integrations

Summary:
Backend:

• Add metric table and API for uptime/response_time/error_rate with daily summaries
• Add subscriber system with email (Nodemailer) and webhook notifications
• Add notification dispatch on incident create/update/resolve
• Add incident-service and maintenance-service linking via service_ids
• Add inbound monitoring webhook endpoint (POST /api/webhooks/ingest)

Frontend:

• Add UptimeChart component (Chart.js 30-day bar chart)
• Add SubscribeForm component (email/webhook subscription)
• Add metricsApi, subscribersApi, and useMetrics composable

Reviewers: ieli

Reviewed By: ieli

Differential Revision: https://devcentral.nasqueron.org/D3965