Record SoH with mileage and display history chart in vehicle details

Automatically collect SoH and mileage, say max once per day if possible (or once er connection to OBD dongle?). Store history and display charge of SoH against mileage in vehicle details tab. Perhaps include a normalise line predicting the SoH change over time going forward.

No files or tests are named. Start by tracing the vehicle details tab and the OBD dongle connection path, then identify how vehicle data is collected and stored. Done should define the collection cadence, persist SoH and mileage history, display the SoH-versus-mileage chart, and clarify whether the predictive normalisation line is required.

Written by the indexing model from the issue text.