On publish failure, enqueue the MAC in a dedup'd in-memory set and let a
background loop retry with the configured backoff sequence. Reconnects
signal an immediate drain so a transient broker outage no longer drops
the update.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Persistent MQTTnet v4 IMqttClient wrapped in a BackgroundService:
attempts to connect on StartAsync (logging and continuing on failure)
and reconnects on a 5s loop while running. Exposes IsConnected so
6.4 can wire it into /health later. Honors username/password and
UseTls from MqttOptions; clean session is disabled.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
