feat: implement sensor calibration flow

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);
 }