M8-R01: add configurable MQTT client identity

app/config.py

@@ -1,7 +1,8 @@
 from functools import lru_cache
 from pathlib import Path
+import re
 
-from pydantic import computed_field
+from pydantic import computed_field, field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -53,6 +54,7 @@ class Settings(BaseSettings):
     mqtt_username: str = ""
     mqtt_password: str = ""
     mqtt_tls_enabled: bool = False
+    mqtt_client_id: str = "home-automation"
 
     # Home Assistant MQTT Discovery (T08 wires into CONFIG_FIELDS/UI; T11 does publishing).
     ha_discovery_enabled: bool = False
@@ -81,6 +83,17 @@ class Settings(BaseSettings):
         extra="ignore",
     )
 
+    @field_validator("mqtt_client_id", mode="before")
+    @classmethod
+    def validate_mqtt_client_id(cls, value: object) -> str:
+        """Normalize a broker-safe base client identity used by every MQTT client."""
+        if not isinstance(value, str):
+            raise ValueError("MQTT client ID must be a string")
+        normalized = value.strip()
+        if not re.fullmatch(r"[A-Za-z0-9][A-Za-z0-9_-]{0,63}", normalized):
+            raise ValueError("MQTT client ID must be a non-empty ASCII slug")
+        return normalized
+
     @computed_field
     @property
     def is_development(self) -> bool: