docs: generate Prometheus-driven monitoring docs and Mermaid diagrams

docs/monitoring-coverage.md

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+# Monitoring Coverage
+
+## Overview
+
+This page is generated from Prometheus-observed runtime inventory. It supplements declared architecture docs and does not replace static source-of-truth configuration.
+
+- Inventory timestamp: `2026-04-13T06:36:45Z`
+- Prometheus URL: `http://prometheus:9090`
+- Active scrape targets observed: `19`
+- Unhealthy scrape targets observed: `0`
+
+## Coverage by job
+
+| job | active targets | unhealthy targets |
+| --- | --- | --- |
+| container-updates | 2 | 0 |
+| kuma | 2 | 0 |
+| node | 7 | 0 |
+| pihole | 1 | 0 |
+| prometheus | 1 | 0 |
+| proxmox-storage | 2 | 0 |
+| telegraf | 2 | 0 |
+| traefik | 2 | 0 |
+
+## Coverage by instance
+
+| instance | jobs | health |
+| --- | --- | --- |
+| docker-update-exporter:9105 | container-updates | 1/1 up |
+| kuma.lan.ddnsgeek.com | kuma | 1/1 up |
+| monitor-kuma:3001 | kuma | 1/1 up |
+| nix-cache:9100 | node | 1/1 up |
+| node-exporter:9100 | node | 1/1 up |
+| pbs.sweet.home:9100 | node | 1/1 up |
+| pbs.sweet.home:9102 | proxmox-storage | 1/1 up |
+| pihole-exporter:9617 | pihole | 1/1 up |
+| pihole:9100 | node | 1/1 up |
+| prometheus:9090 | prometheus | 1/1 up |
+| pve.sweet.home:9100 | node | 1/1 up |
+| pve.sweet.home:9101 | proxmox-storage | 1/1 up |
+| raspberrypi.tail13f623.ts.net:8080 | traefik | 1/1 up |
+| raspberrypi.tail13f623.ts.net:9100 | node | 1/1 up |
+| raspberrypi.tail13f623.ts.net:9105 | container-updates | 1/1 up |
+| raspberrypi.tail13f623.ts.net:9273 | telegraf | 1/1 up |
+| server:9100 | node | 1/1 up |
+| telegraf:9273 | telegraf | 1/1 up |
+| traefik.lan.ddnsgeek.com:8080 | traefik | 1/1 up |
+
+## Coverage by service
+
+| service | instances | health |
+| --- | --- | --- |
+| unknown | docker-update-exporter:9105, kuma.lan.ddnsgeek.com, monitor-kuma:3001, nix-cache:9100, node-exporter:9100, pbs.sweet.home:9100, pbs.sweet.home:9102, pihole-exporter:9617, pihole:9100, prometheus:9090, pve.sweet.home:9100, pve.sweet.home:9101, raspberrypi.tail13f623.ts.net:8080, raspberrypi.tail13f623.ts.net:9100, raspberrypi.tail13f623.ts.net:9105, raspberrypi.tail13f623.ts.net:9273, server:9100, telegraf:9273, traefik.lan.ddnsgeek.com:8080 | 19/19 up |
+
+## Unhealthy targets
+
+| job | instance | scrape URL | health | last error |
+| --- | --- | --- | --- | --- |
+| none |  |  |  |  |
+
+## Unknowns / missing metadata
+
+| label | targets missing |
+| --- | --- |
+| exposure | 19 |
+| hostname | 19 |
+| hypervisor | 19 |
+| network | 19 |
+| service | 19 |
+
+Unknown or missing metadata is treated as `unknown` in generated summaries to avoid over-claiming topology.
+
+## Regeneration instructions
+
+```bash
+python3 scripts/render_prometheus_docs.py --inventory-file docs/runtime/prometheus-inventory.json
+```