Track users who join monitored channels and only announce their
departure when they disconnect. This prevents announcing departures
for users who never joined channels we care about.

The `tail -n 0` option ensures that the script only processes new lines
appended to the log file after the script has started, preventing
re-processing of existing file content on startup.

The Mumble server can assign negative user session IDs (SIDs), e.g.,
`(-1)`. The regex patterns for parsing both move and disconnect events
have been updated to correctly handle negative session IDs.

Additionally, a new `log()` function provides granular debug output when
the `DEBUG` environment variable is set to a truthy value.