fix: server odometer stat shows 0 when no odometer data exists

COALESCE(MAX(odometer), 0) returned 0 when all tracking rows have NULL
odometer (expected before the car is connected — PID 0xA6 likely unsupported
on 2013 Yaris). Now returns NULL which the overview JS renders as '—'.

Co-authored-by: Copilot <[email protected]>

server/modules/db.py

@@ -379,7 +379,7 @@ def get_overview_stats(device_id):
                 (SELECT COALESCE(SUM(duration_s), 0) FROM trips WHERE device_id = %s) AS total_s,
                 (SELECT COALESCE(MAX(max_speed_kmh), 0) FROM trips WHERE device_id = %s) AS top_speed,
                 (SELECT COUNT(*) FROM dashcam_files WHERE device_id = %s) AS video_count,
-                (SELECT COALESCE(MAX(odometer), 0) FROM tracking_points WHERE device_id = %s) AS odometer_km
+                (SELECT MAX(odometer) FROM tracking_points WHERE device_id = %s AND odometer IS NOT NULL) AS odometer_km
             """,
             (device_id,) * 6,
         )
@@ -390,5 +390,5 @@ def get_overview_stats(device_id):
         "total_drive_time_s": int(r[2]),
         "top_speed_kmh": float(r[3]),
         "video_count": r[4],
-        "odometer_km": float(r[5]),
+        "odometer_km": float(r[5]) if r[5] is not None else None,
     }