ci: optional Prometheus snmp_exporter integration (Phase 12)

[strausmann]

Goal

Optional integration with snmp_exporter to expose printer metrics in Prometheus. Hub itself stays unchanged — exporter scrapes the printers directly.

Why optional

Phase 0 confirmed all the OIDs we need (display, page counter, alerts) respond on Brother PT/QL printers. snmp_exporter with Brother generator config can produce Prometheus metrics without any hub-side changes.

Tasks

• Sample prometheus.yml snippet in examples/
• Brother SNMP module config for snmp-exporter generator
• Sample Grafana dashboard JSON (page counter, status heatmap)
• Document in docs/architecture.md as add-on
• Sample alerts (e.g. printer_status != "ready" for >5min)

Out of scope

• Embedding metrics endpoint inside the hub container
• Replacing the hub's own status flow