Trips starting/ending on speed-only rows violated the trips.*GPSX NOT NULL
constraint. Use the first/last GPS-bearing row for start/end coordinates, and
skip a trip that never got any GPS fix (no location to record).
- _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
Mathieu Moran