Webhook or MQTT support for specific actions in Core

[AlexeyBoiler]

In some use cases, it is necessary to receive a notification from Core when specific events occur (e.g. stream started, stream stopped, scene change detected, etc.).

We propose adding support for either: • Webhooks (HTTP callbacks to a configurable endpoint), or • MQTT messages (published to a configurable topic and broker)

This would allow external systems to react to stream-related events in real-time and integrate Core into a wider event-driven architecture.