ibsystem2mqtt_v5.py previously subscribed to ibsystem/+/+/set/# only
once at startup. When paho-mqtt auto-reconnected (e.g. TCP keep-alive
timeout after 3 days), subscriptions were NOT restored, causing all
switch commands to be silently dropped (messages_received stayed at 15).

Fix: Added on_connect callback that resubscribes on every (re)connection.
Tested: commands_executed: 2, messages_received: 2 after restart.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>