feat: log OBD speed before GPS fix so rolling starts aren't lost

- _log_loop: when GPS has no fix but OBD speed > 1, write speed-only rows
  (NULL coordinates) instead of dropping the motion
- insert_tracking: accept None gps_x/gps_y -> NULL GPSX/GPSY
- app_trip: keep NULL-coord rows through normalize; _segment returns 0 for
  missing coords; route_points drops them (no position to draw)
- tests: speed-only rows build a trip with max speed + zero distance
- docs: FEAT-12 + readme note; server already filters NULL-GPS safely

modules/app_sql.py

@@ -314,7 +314,11 @@ def count_active_dtcs():
 
 
 def insert_tracking(gps_x, gps_y, speed, odometer=None):
-    """Insert a single trip-tracking snapshot row (GPS position + speed)."""
+    """Insert a single trip-tracking snapshot row (GPS position + speed).
+
+    gps_x/gps_y may be None (logged before the first GPS fix) — the row then
+    stores NULL coordinates and relies on OBD speed for motion detection.
+    """
     conn = get_conn()
     if conn is None:
         return False
@@ -323,8 +327,14 @@ def insert_tracking(gps_x, gps_y, speed, odometer=None):
              VALUES (?, ?, ?, ?, ?, ?, ?)"""
     with _conn_lock:
         try:
-            latitude = float(str(gps_x).replace("\x00", "").strip())
-            longitude = float(str(gps_y).replace("\x00", "").strip())
+            latitude = (
+                None if gps_x is None
+                else float(str(gps_x).replace("\x00", "").strip())
+            )
+            longitude = (
+                None if gps_y is None
+                else float(str(gps_y).replace("\x00", "").strip())
+            )
             conn.execute(
                 sql,
                 (car.model, account.name, datetime.now(), latitude, longitude, speed, odometer),
@@ -847,6 +857,12 @@ def _log_loop():
                 ) and insert_tracking(position[0], position[1], speed, app_obd.odometer):
                     last_position = position
                     last_logged_at = now
+        elif speed is not None and speed > 1:
+            # No GPS fix yet but the car is moving (OBD speed) — log a
+            # speed-only row so a rolling start / mid-drive reboot isn't lost.
+            # Coordinates stay NULL until the first fix; trip building treats
+            # these as OBD-motion events with zero GPS distance.
+            insert_tracking(None, None, speed, app_obd.odometer)
 
         time.sleep(LOG_INTERVAL)