feat: implement sensor calibration flow

lib/data/repositories/phoenix_sensor_repository.dart

@@ -59,4 +59,36 @@ class PhoenixSensorRepository implements SensorRepository {
   Stream<Map<String, dynamic>> sensorEvents() => realtime
       .channelMessages('sensors')
       .map((m) => {'event': m.event, ...m.payload});
+
+  @override
+  Future<int> beginCalibration(int id) async {
+    final json = await client.beginCalibration(id);
+    return json['samples_needed'] as int;
+  }
+
+  @override
+  Future<void> startStage(int id, double distance) =>
+      client.startStage(id, distance);
+
+  @override
+  Future<({double rssiRef, double pathLossExp})> finishCalibration(
+      int id) async {
+    final json = await client.finishCalibration(id);
+    return (
+      rssiRef: (json['rssi_ref'] as num).toDouble(),
+      pathLossExp: (json['path_loss_exp'] as num).toDouble(),
+    );
+  }
+
+  @override
+  Future<void> cancelCalibration(int id) => client.cancelCalibration(id);
+
+  @override
+  Stream<({String event, Map<String, dynamic> payload})> calibrationEvents(
+          String sensorDeviceId) =>
+      realtime.channelMessages('calibration:$sensorDeviceId');
+
+  @override
+  void leaveCalibrationChannel(String sensorDeviceId) =>
+      realtime.leaveChannel('calibration:$sensorDeviceId');
 }

lib/data/repositories/sensor_repository.dart

@@ -16,4 +16,30 @@ abstract class SensorRepository {
   /// Stream of raw SensorsChannel messages. Each map contains an `event` key
   /// (`sensor_announced` or `sensor_enrollment_timeout`) plus the payload.
   Stream<Map<String, dynamic>> sensorEvents();
+
+  // ---------------------------------------------------------------------------
+  // Calibration
+  // ---------------------------------------------------------------------------
+
+  /// Enters calibration mode on the sensor. Returns the number of samples the
+  /// server will collect per stage.
+  Future<int> beginCalibration(int id);
+
+  /// Starts a collection stage at [distance] metres.
+  Future<void> startStage(int id, double distance);
+
+  /// Runs least-squares regression over all completed stages and persists the
+  /// result. Returns the fitted (rssiRef, pathLossExp) pair.
+  Future<({double rssiRef, double pathLossExp})> finishCalibration(int id);
+
+  /// Aborts calibration and discards all accumulated stage data.
+  Future<void> cancelCalibration(int id);
+
+  /// Real-time events from the server's CalibrationChannel.
+  /// Topic: `calibration:{sensorDeviceId}` (the string BLE device ID).
+  Stream<({String event, Map<String, dynamic> payload})> calibrationEvents(
+      String sensorDeviceId);
+
+  /// Leaves the calibration Phoenix channel for [sensorDeviceId].
+  void leaveCalibrationChannel(String sensorDeviceId);
 }

lib/data/sources/localiser/realtime_data_client.dart

@@ -78,6 +78,12 @@ class RealtimeDataClient {
         .map((msg) => (event: msg.event.value, payload: msg.payload ?? const {}));
   }
 
+  /// Leaves [topic] and removes it from the channel cache.
+  void leaveChannel(String topic) {
+    final ch = _channels.remove(topic);
+    ch?.leave();
+  }
+
   /// Pushes [event] on [topic] and waits for the server reply.
   /// The channel must have been joined first via [channel].
   Future<Map<String, dynamic>> push(

lib/data/sources/localiser/sensor_client.dart

@@ -38,17 +38,22 @@ class SensorClient extends LocaliserdClient {
           })
           as Map<String, dynamic>;
 
-  Future<Map<String, dynamic>> startCalibration(
-    int id,
-    double referenceDistance,
-  ) async =>
-      await post('/api/sensors/$id/calibration/start', {
-            'reference_distance': referenceDistance,
+  Future<Map<String, dynamic>> beginCalibration(int id) async =>
+      await post('/api/sensors/$id/calibration/begin') as Map<String, dynamic>;
+
+  Future<Map<String, dynamic>> startStage(int id, double distance) async =>
+      await post('/api/sensors/$id/calibration/stage', {
+            'distance': distance,
           })
           as Map<String, dynamic>;
 
-  Future<Map<String, dynamic>> stopCalibration(int id) async =>
-      await post('/api/sensors/$id/calibration/stop') as Map<String, dynamic>;
+  Future<Map<String, dynamic>> finishCalibration(int id) async =>
+      await post(
+        '/api/sensors/$id/calibration/finish',
+      ) as Map<String, dynamic>;
+
+  Future<void> cancelCalibration(int id) =>
+      delete('/api/sensors/$id/calibration');
 
   Future<String> getVersion(int id) async {
     final response =