feat(monitoring): expose backend version in Prometheus

[alanpeixinho]

What it is

• Passes DASHBOARD_VERSION to the backend container
• Exposes it in Prometheus as dashboard_build_info for deploy tracking
• Adds a Running Version panel and Deploys annotation to the API Grafana dashboard

Closes #1591

How to test

1. Start monitoring and the backend with PROMETHEUS_METRICS_ENABLED=true and DASHBOARD_VERSION=test-a
2. Check http://localhost:8001/metrics/ for dashboard_build_info{version="test-a"}
3. Import monitoring/dashboard.json in Grafana — confirm Running Version shows test-a
4. Restart the backend with DASHBOARD_VERSION=test-b — confirm the version updates and a deploy annotation appears

• pass DASHBOARD_VERSION to the backend container
• report it as a dashboard_build_info marker for deploy tracking

Closes #1591