feat(hunt): track walking distance via GPS, show live locations on map

Add foreground-only GPS distance tracking for hunts

server/utils/distance.ts

@@ -0,0 +1,22 @@
+/**
+ * Team walking-distance aggregation.
+ *
+ * A team's distance is the average of the `UserTrack.distanceMeters` of its
+ * _current_ participants (the team owner plus its team members) that actually
+ * recorded a distance. Participants without a recorded track are excluded
+ * rather than diluting the average with zeros.
+ */
+export function teamAverageDistance(
+  participantIds: number[],
+  tracks: Map<number, number>
+): number {
+  const unique = [...new Set(participantIds)];
+  const distances = unique
+    .map((id) => tracks.get(id))
+    .filter((d): d is number => d !== undefined);
+
+  if (distances.length === 0) return 0;
+
+  const sum = distances.reduce((acc, d) => acc + d, 0);
+  return Math.round(sum / distances.length);
+}