Add Home Assistant MQTT Discovery support

[OKDaG]

Summary

Adds opt-in Home Assistant MQTT Discovery to the existing MQTT client. When enabled,
vzlogger publishes retained discovery configs (<prefix>/sensor/vzlogger_<uuid>/config)
for each channel on first reading, letting Home Assistant auto-create sensors with the
correct unit_of_measurement, device_class and state_class derived from the
channel's OBIS identifier.

Why

Today every Home Assistant user running vzlogger needs to hand-write a YAML sensor
block per channel just to give the raw smartmeter/reading/chnN/raw topic a unit
and a meaningful name. Doing this once inside vzlogger removes that boilerplate and
makes the readings show up in HA as proper Energy / Power / Voltage / Current sensors
out of the box.

Design notes

• Opt-in. ha_discovery.enabled defaults to false — no behaviour change for
  existing setups.
• No new dependencies. Discovery JSON is built with the already-linked libjson-c.
• Lazy publish. The discovery config is sent once per channel, hooked next to the
  existing per-channel announce flow in MqttClient::publish(). Channels without
  readings never produce discovery topics.
• Static OBIS map with 18 common SML codes (1.8.0/1.8.1/1.8.2, 2.8.0/2.8.1/2.8.2,
  16.7.0, 1.7.0, 2.7.0, 36/56/76.7.0, 32/52/72.7.0, 31/51/71.7.0). Unknown OBIS codes
  fall back to a unit-less generic sensor.
• value_template is only emitted when mqtt.timestamp=true (JSON payload);
  otherwise the raw numeric payload is consumed directly by HA.
• Separate translation unit (src/mqtt_ha_discovery.cpp) for the metadata lookup
  and config-JSON assembly so they can be unit-tested without a broker.
• One HA device per vzlogger process (vzlogger by default, configurable via
  device_name / device_identifier). Per-meter grouping would require a
  Channel→Meter back-reference and is intentionally left as a follow-up to keep
  this PR contained.

Config

"mqtt": {
    "enabled": true,
    "host": "…",
    "topic": "smartmeter/reading",
    "ha_discovery": {
        "enabled": true,             // opt in
        "prefix": "homeassistant",   // optional, HA discovery prefix
        "device_name": "vzlogger",   // optional, HA device name
        "device_identifier": ""      // optional, defaults to mqtt.id or device_name
    }
}

Tests

Adds tests/ut_mqtt_ha_discovery.cpp with 6 gtest cases covering:

• Lookup of known OBIS metadata (energy import, active power)
• Lookup of unknown OBIS returns nullptr
• Discovery-JSON shape for a known OBIS (verifies name, unique_id, state_topic,
  unit_of_measurement, device_class, state_class, device.*)
• Unknown-meta path omits unit_of_measurement, device_class, state_class
• timestamp: true adds the value_template

All existing tests still pass.

Test plan

• docker build --build-arg build_test=on . succeeds
• vzlogger_unit_tests, mock_metermap, mock_MeterW1therm, mock_MeterOMS,
  mock_MeterS0 — 5/5 passing via make test
• MqttHaDiscovery.* — 6/6 passing
• ./check-formatting.sh clean (verified in an ubuntu:latest container with
  clang-format-20)
• Smoke-tested against a real Mosquitto broker and Home Assistant 2025.x; sensors
  appear under the vzlogger device with correct units (Wh / W) and the Energy
  Dashboard picks up the total_increasing energy sensors.

[r00t-]

thanks for your contribution!

• this very much looks like it was at least partially generated with an LLM, i don't know if other project members have an opinion, but i would personally very much prefer if that was noted/detailed in the commits and MR
• not your fault, but it would be so much better if mqtt was an api instead of hardcoded, so this code could live in an MQTT api instead of in the root (see mqtt should be implemented as an API, not hardcoded into vzlogger core #550 )
  (that would probably also solve the "One HA device per vzlogger process" issue.)
• i'm personally not a user of Home Assistant or mqtt and won't be able to test this. any takers?

[OKDaG]

LLM was partly involved creating the PR text.
LLM was involved creating the PR for issue #550

[OKDaG]

Will adjust this PR if 550 is accepted.