EVALink event notification mechanism

Part of a general event notification subsystem.

Create a listener process on the MQTT broker log, along with a defined event syntax format. Then different events could be subscribed to by different users for different purposes. Examples:

- all MQTT traffic --> dedicated Discord "MQTT log" channel for monitoring
- MQTT traffic of non-recognized device ID's --> Discord channel for alerts and warnings
- MQTT traffic during certain date ranges ---> Discord channel requested by a given crew
- Temperature telemetry outside of "normal" ranges ---> new Ops dashboard area

etc

No files, tests, or entry points are named. Start by mapping the EVALink service and how it currently accesses the MQTT broker log, then clarify the event syntax and subscription boundaries using the Discord, alert, dashboard, and date-range examples. Done means the listener, event format, and subscriber behavior are specified well enough to implement and verify.

Written by the indexing model from the issue text.