fix: Sensor schema drift with upstream API

lib/domain/models/sensor.dart

@@ -6,16 +6,14 @@ class Sensor {
     this.roomId,
     this.x,
     this.y,
-    this.rssiRef,
   });
 
   final int id;
   final String sensorId; // BLE MAC / provisioning device ID
   final String? name;    // human-readable label
   final int? roomId;
-  final double? x;       // floor_x from API
-  final double? y;       // floor_y from API
-  final double? rssiRef;
+  final double? x;       // room relative
+  final double? y;       // room relative
 
   bool get isPlaced => roomId != null;
   String get displayName => name ?? sensorId;
@@ -25,9 +23,8 @@ class Sensor {
         sensorId: json['sensor_id'] as String,
         name: json['name'] as String?,
         roomId: json['room_id'] as int?,
-        x: (json['floor_x'] as num?)?.toDouble(),
-        y: (json['floor_y'] as num?)?.toDouble(),
-        rssiRef: (json['rssi_ref'] as num?)?.toDouble(),
+        x: (json['x'] as num?)?.toDouble(),
+        y: (json['y'] as num?)?.toDouble(),
       );
 
   Sensor copyWith({
@@ -44,6 +41,5 @@ class Sensor {
         roomId: roomId ?? this.roomId,
         x: x ?? this.x,
         y: y ?? this.y,
-        rssiRef: rssiRef ?? this.rssiRef,
       );
 }