ha_discovery: publish state/availability under home_automation prefix

Follow the standard MQTT Discovery split: discovery config stays under the HA
discovery prefix (homeassistant/.../config, required), but the actual state and
availability are published under our own prefix (default home_automation/). The
config payload's state_topic/availability fields point at the new prefix so HA
reads state from there.

- new config ha_state_topic_prefix (default home_automation).
- build_discovery_payload + all publish_* split discovery_prefix (config topic)
  from state_prefix (state/availability). energy-cost still omits availability;
  modbus availability structure unchanged. Tests updated.

app/config.py

@@ -57,6 +57,9 @@ class Settings(BaseSettings):
     # Home Assistant MQTT Discovery (T08 wires into CONFIG_FIELDS/UI; T11 does publishing).
     ha_discovery_enabled: bool = False
     ha_discovery_prefix: str = "homeassistant"
+    # State/availability topics use a separate prefix so they live outside the HA discovery
+    # namespace.  HA still receives state via the state_topic declared in the discovery config.
+    ha_state_topic_prefix: str = "home_automation"
 
     # DSMR smart-meter ingest via MQTT (M6; default off — opt-in).
     # Subscribe to dsmr_mqtt_topic when dsmr_ingest_enabled=True.