feat: implement watchPositions() tag position stream

lib/domain/models/tag.dart

@@ -8,11 +8,13 @@ class Tag {
     this.currentRoomId,
     this.lastPosition,
     this.lastSeen,
+    this.color,
   });
 
   final int id;
   final String tagId; // BLE beacon advertised identifier
   final String name;
+  final String? color;
   final String? currentRoomId;
   final Position? lastPosition;
   final DateTime? lastSeen;
@@ -34,6 +36,7 @@ class Tag {
         'current_room_id': currentRoomId,
         'last_position': lastPosition?.toJson(),
         'last_seen': lastSeen?.toIso8601String(),
+        'color': color,
       };
 
   factory Tag.fromJson(Map<String, dynamic> json) => Tag(
@@ -41,6 +44,7 @@ class Tag {
         tagId: json['tag_id'] as String,
         name: json['name'] as String,
         currentRoomId: json['current_room_id'] as String?,
+        color: json['color'] as String?,
         lastPosition: json['last_position'] == null
             ? null
             : Position.fromJson(json['last_position'] as Map<String, dynamic>),