fix: resolve null pointer and serialization errors in /watch endpoint

bin/routes/auth_routes.dart

@@ -60,8 +60,12 @@ class AuthRoutes {
   }
 
   Future<Response> _watch(Request request, String login) async {
-    final uniqueId = request.url.queryParameters['unique_id'];
-    final geoId = database.getGeoIdByShareId(uniqueId!);
+    final uniqueId = request.url.queryParameters['share_id'];
+    if (uniqueId == null) {
+      return Response(404, body: jsonEncode({'error': 'Share link not found'}), headers: {'Content-Type': 'application/json'});
+    }
+
+    final geoId = database.getGeoIdByShareId(uniqueId);
 
     if (geoId == null) {
       await database.createLog(login, 'Accessed invalid share link');
@@ -79,8 +83,8 @@ class AuthRoutes {
     return Response(200, body: jsonEncode({
       'x': position.xValue,
       'y': position.yValue,
-      'last_update': position.lastUpdate,
-      'expires_at': position.expiresAt,
+      'last_update': position.lastUpdate.toIso8601String(),
+      'expires_at': position.expiresAt.toIso8601String(),
     }), headers: {'Content-Type': 'application/json'});
   }
 }