fix: rewrite GPS stats to calculate from stored history (#22)

- Replace Traccar summary API dependency with local Haversine distance calculation
- Calculate mileage from GpsLocationHistory table (sum consecutive positions)
- Filter out GPS jitter (<0.01mi), gaps (>10min), and unrealistic speeds (>100mph)
- Calculate trips, driving time, average/top speed from position history
- Add detailed stats logging for debugging

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/prisma/schema.prisma

@@ -420,7 +420,7 @@ model GpsLocationHistory {
   latitude        Float
   longitude       Float
   altitude        Float?
-  speed           Float?    // km/h
+  speed           Float?    // mph (converted from knots during sync)
   course          Float?    // Bearing in degrees
   accuracy        Float?    // Meters
   battery         Float?    // Battery percentage (0-100)