notifications.md: full 7-step audit

- Replaced mermaid diagram with real Nextcloud diagram; rebuild caught a
  real bug in the old one (showed Kopia/Bazarr flowing through the App
  Router, which flows.json disproves - they bypass it entirely)
- Live-verified Mosquitto broker config/ACL, Node-RED flow routing (16
  switch rules = 15 apps + unknown fallback, 2 dedicated endpoints),
  mqtt-monitor.sh check count against the real script
- Named NetBox and Kestra directly (previously generic) after the real
  Node-RED screenshot made them visible anyway; Postiz/nocodb-content
  stay generic per standing instruction
- Added real evidence: routing-table command output, the real
  topic-agnostic Inventory Parse function source, real Mosquitto
  ACL/config, real check-count grep, a real Node-RED flow canvas
  screenshot, and a real Mosquitto connection log screenshot (the
  kestra-notify client is live proof of the MQTT-not-HTTP claim)
- KB fix: Ansible mqtt_client Role note was missing a 23rd real check
  (crash-looping systemd units, distinct from the failed-units check,
  added to the script after a real vault-agent crash-loop incident)
- Sanitize-checked: no internal IPs/domains/personal info/forbidden app
  names

assets/notifications-fan-in-fan-out.svg

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