Fix: Enable WebSocket updates for custom Modbus register devices

[JakubRanosz]

Fix: Enable WebSocket updates for custom Modbus register devices

Closes #214

Problem

Modbus register devices (e.g., temperature sensors connected via RS485) were not sending value updates via WebSocket, even though:

• Values could be successfully read via REST API
• Communication with the device was working correctly
• Other device types (AI, DI, RO, etc.) were sending updates as expected

This was a regression from Evok v2, where Modbus register updates were transmitted correctly.

Root Cause

The Register class was missing the necessary infrastructure to participate in the event notification system:

1. No check_new_data() method to detect value changes
2. Not registered as an "eventable device" during initialization
3. No cached value tracking for change detection

As a result, the periodic scan loop would read register values but never detect changes or notify WebSocket clients.

Solution

This PR adds the missing event notification infrastructure to the Register class:

Changes Made

1. Added value caching (_cached_value)

   • Stores the last read value for comparison

2. Implemented check_new_data() method

   • Compares current value with cached value
   • Returns True when a change is detected
   • Updates the cached value for next comparison

3. Registered Register devices as eventable

   • Added self.__register_eventable_device(_reg) in parse_feature_register()
   • Ensures Register devices are included in the scan loop's change detection

4. Updated value property

   • Now uses cached value when available for consistency
   • Falls back to reading from hardware if cache is empty

Technical Details

The fix follows the same pattern used by other eventable devices (DigitalInput, DataPoint, etc.):

• Store previous value
• Compare on each scan cycle
• Notify via WebSocket when changes occur