feat: add SunEnergyXT 500 Series battery storage module

[ChristophCaina]

Summary

This PR adds support for the SunEnergyXT 500 Series (including the 500 PRO) battery storage system to openWB.

Communication

The device exposes a local HTTP API — no cloud dependency, no Modbus:

Endpoint	Method	Description
http://<IP>/read	GET	Read all device state fields as JSON
http://<IP>/write	POST	Write control parameters

Features

Read (passive monitoring)

• Battery SoC (SC)
• Battery power — positive = charging, negative = discharging (PB)
• Grid power (GP)
• PV power (PP)
• Max inverter power (IS)

Active control (power_limit_controllable = True)

The module supports full active battery control by openWB:

openWB request	Device command	Effect
None — automatic	MM=1, GS=0	Device self-regulates (self-consumption mode)
0 — stop discharge	MM=0, GS=0	Battery inactive
> 0 — discharge N W	MM=0, GS=+N	Targeted discharge
< 0 — charge N W	MM=0, GS=-N	Forced charging

Configuration

• IP address — local IP of the SunEnergyXT device
• Port — default 80
• Timeout — HTTP timeout in seconds, default 5

UI

A Vue configuration page is provided via a separate PR to openwb-ui-settings: ChristophCaina/openwb-ui-settings

Testing

• ✅ Tested with SunEnergyXT device simulator
• ✅ SoC, charge/discharge power verified against Home Assistant integration
• ✅ Active control (power_limit_controllable) recognized by openWB
• ⏳ Test with real hardware pending (EVSE issue on physical openWB being resolved and storage is available)
• ⏳ Active power limit control (set_power_limit) not yet verified —
  requires a real charging session with physical hardware