Add persistent car performance analytics

CAR_TELEMETRY.md

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+# Car telemetry and retention
+
+The Pi samples the MG4 every 10 seconds while ADB is enabled. Speed, gear, and steering
+angle come from the verified ArcSoft properties. Battery values come from the read-only
+content provider in `cloud.molberg.mgutility`:
+
+```bash
+adb shell content call \
+  --uri content://cloud.molberg.mgutility.vehicle \
+  --method sample
+```
+
+The provider must be present in the installed MG Utility APK. Until it connects to the
+SAIC vehicle service, battery fields remain unavailable while the ArcSoft signals and
+their history continue working.
+
+## Storage limits
+
+- Raw 10-second samples are retained for 90 days and capped at 800,000 rows.
+- Hourly rollups are retained for 10 years and capped at 88,000 rows.
+- Lifetime counters and observed records survive raw-data pruning.
+- Graph endpoints use indexed time buckets and return compact aggregates rather than
+  every raw sample.
+
+At the configured interval the raw cap is approximately 90 days. Expected storage is
+well below 150 MB, although SQLite page allocation and filesystem details can vary.
+
+## Derived statistics
+
+- Tracked distance integrates consecutive speed samples no more than 60 seconds apart.
+- The dashboard's range estimate uses tracked distance per observed SOC consumed. It
+  waits for at least 10 km and 5 percentage points of discharge before showing a value.
+- Charging power is an estimate based on SOC gain over a 2-10 minute window and the
+  MG4 Luxury 2022 usable-capacity assumption of 61.7 kWh. It is deliberately labeled as
+  estimated and is capped at 350 kW to reject corrupt samples.
+- Peak values mean the highest value observed at a 10-second sampling point. They are
+  not guaranteed to capture a shorter event between samples.
+
+The collector sends no vehicle-control calls. Installing MG Utility and enabling ADB
+remain parked-vehicle administration tasks.