Make leave notification optional via configuration

Currently, the bot sends a notification when someone leaves a voice channel - either to a single channel or a dedicated leave channel (based on env config). While this is useful in some contexts, it may be unnecessary or noisy for others.

### Request
Add a configuration option to enable or disable leave notifications.

### Goals

* Allow users to opt out of leave messages when configuring the bot
* Default to the current behavior (enabled), unless explicitly disabled
* Keep configuration simple and intuitive

### Suggested Implementation
* Check flag before sending leave notifications
* Document the new option in the README or config template for both configuration options

### Why?

* Provides flexibility for different communities or deployment needs
* Makes the bot less noisy in low-attention or small-group settings

Happy to review PRs for this - feel free to comment if you're interested!