fix: smooth GPS-derived replay speed to kill jitter spikes

route_points (Pi) and get_trip_route (server) derived GPS speed from a
single 5s segment, so GPS position jitter could read 150+ km/h when the car
wasn't going that fast. Both now average over a trailing ~30s window (same
as the max-speed calc), so spikes are damped.

server/modules/db.py

@@ -625,6 +625,7 @@ def get_trip_route(device_id, trip_id):
     result_points = []
     cumulative = 0.0
     prev = None
+    gps_window = []  # trailing (lat, lon, datetime) points for GPS speed smoothing
     for p in points:
         lat = float(p[1])
         lon = float(p[2])
@@ -632,18 +633,24 @@ def get_trip_route(device_id, trip_id):
         raw_odo = p[4]
         speed_source = "obd" if speed > 0 else "gps"
         # Mirror the Pi's route_points GPS-speed fallback: when OBD speed is
-        # missing (adapter dropout), derive speed from the previous point's
-        # movement so the replay doesn't flatline at 0 km/h while the car moves.
-        if speed <= 0 and prev is not None:
-            seconds = (p[0] - prev[2]).total_seconds()
+        # missing (adapter dropout), derive speed from a trailing ~30s window
+        # of movement (not the previous point) so the replay doesn't flatline
+        # at 0 km/h while the car moves — and a single jittery 5s segment can't
+        # spike the readout.
+        if speed <= 0 and gps_window:
+            start = gps_window[0]
+            seconds = (p[0] - start[2]).total_seconds()
             if seconds > 0:
-                dist_km = hav(prev[0], prev[1], lat, lon)
-                seg_speed = dist_km / seconds * 3600
-                if seg_speed <= 220 and dist_km * 1000 >= 8:
-                    speed = seg_speed
+                dist_km = hav(start[0], start[1], lat, lon)
+                w_speed = dist_km / seconds * 3600
+                if w_speed <= 220 and dist_km * 1000 >= 8:
+                    speed = w_speed
         if prev is not None:
             cumulative += hav(prev[0], prev[1], lat, lon)
         prev = (lat, lon, p[0])
+        gps_window.append((lat, lon, p[0]))
+        if len(gps_window) > 6:
+            gps_window.pop(0)
         if raw_odo is not None and float(raw_odo) > 0:
             odometer = float(raw_odo)
         else: