M5: default Modbus polling to off (opt-in) to avoid unnecessary DB writes

app/config.py

@@ -42,7 +42,9 @@ class Settings(BaseSettings):
     auth_totp_issuer: str = ""  # defaults to app_name when empty
 
     # Modbus polling — global kill-switch (CONFIG_FIELDS registered in T08).
-    modbus_polling_enabled: bool = True
+    # Off by default: polling is opt-in so a fresh deploy writes no modbus_reading
+    # rows until the admin explicitly enables it (matches mqtt/discovery defaults).
+    modbus_polling_enabled: bool = False
 
     # MQTT broker connection (T08 wires into CONFIG_FIELDS/UI; T10 builds the client).
     mqtt_enabled: bool = False