[SENSOR] Create sensor base class with JSON output

[ezehf2]

Create a Python file called sensor_base.py containing a base class that all four sensor types will inherit from. The class should store the sensor type, the room ID, and a method called generate_reading() that returns a properly formatted JSON message containing the sensor type, room, value, unit, and a timestamp. No broker connection is needed at this stage — the class just needs to produce correctly structured output that can later be published via MQTT.