feat: revamp sensor add flow

lib/data/sources/ble/ble_provisioner.dart

@@ -31,26 +31,61 @@ class BleProvisioner {
 
   BluetoothDevice? _connectedDevice;
 
-  Stream<BleScanResult> scan() async* {
-    await _requestScanPermissions();
+  // Continuously scans for nearby ESP32 sensors, restarting after each
+  // 15-second window, until the returned stream is cancelled.
+  Stream<BleScanResult> scan() {
+    StreamSubscription<List<ScanResult>>? resultsSub;
+    StreamSubscription<bool>? stateSub;
+    bool started = false;
+    late StreamController<BleScanResult> controller;
 
-    await FlutterBluePlus.startScan(
-      withServices: [Guid(_serviceUuid)],
-      timeout: const Duration(seconds: 30),
+    Future<void> startScan() async {
+      if (controller.isClosed) return;
+      started = true;
+      try {
+        await FlutterBluePlus.startScan(
+          withServices: [Guid(_serviceUuid)],
+        );
+      } catch (_) {}
+    }
+
+    controller = StreamController<BleScanResult>(
+      onListen: () async {
+        try {
+          await _requestScanPermissions();
+        } catch (e) {
+          controller.addError(e);
+          await controller.close();
+          return;
+        }
+
+        resultsSub = FlutterBluePlus.scanResults.listen((results) {
+          for (final r in results) {
+            final name = r.device.platformName;
+            if (name.startsWith('anchor_') && !controller.isClosed) {
+              controller.add(BleScanResult(
+                deviceId: r.device.remoteId.str,
+                name: name,
+                rssi: r.rssi,
+              ));
+            }
+          }
+        });
+
+        if(!FlutterBluePlus.isScanningNow) {
+          await startScan();
+        }
+      },
+
+      onCancel: () {
+        resultsSub?.cancel();
+        stateSub?.cancel();
+        FlutterBluePlus.stopScan();
+        controller.close();
+      },
     );
 
-    await for (final results in FlutterBluePlus.scanResults) {
-      for (final r in results) {
-        final name = r.device.platformName;
-        if (name.startsWith('anchor_')) {
-          yield BleScanResult(
-            deviceId: r.device.remoteId.str,
-            name: name,
-            rssi: r.rssi,
-          );
-        }
-      }
-    }
+    return controller.stream;
   }
 
   Future<void> stopScan() => FlutterBluePlus.stopScan();

lib/data/sources/localiser/realtime_data_client.dart

@@ -56,6 +56,28 @@ class RealtimeDataClient {
         .map((msg) => msg.payload ?? const {});
   }
 
+  /// Like [channel], but includes the Phoenix event name in each emission.
+  Stream<({String event, Map<String, dynamic> payload})> channelMessages(
+    String topic, {
+    Map<String, dynamic> params = const {},
+  }) {
+    final socket = _socket;
+    if (socket == null) throw StateError('RealtimeDataClient not connected');
+
+    final ch = _channels.putIfAbsent(
+      topic,
+      () {
+        final c = socket.addChannel(topic: topic, parameters: params);
+        c.join();
+        return c;
+      },
+    );
+
+    return ch.messages
+        .where((msg) => msg.event.value != 'phx_reply')
+        .map((msg) => (event: msg.event.value, payload: msg.payload ?? const {}));
+  }
+
   /// 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

@@ -25,6 +25,13 @@ class SensorClient extends LocaliserdClient {
   Future<Map<String, dynamic>> unplaceSensor(int id) async =>
       await deleteBody('/api/sensors/$id/place') as Map<String, dynamic>;
 
+  Future<Map<String, dynamic>> createSensor(String sensorId,
+          {String? name}) async =>
+      await post('/api/sensors', {
+        'sensor_id': sensorId,
+        if (name != null) 'name': name,
+      }) as Map<String, dynamic>;
+
   Future<Map<String, dynamic>> startCalibration(
           int id, double referenceDistance) async =>
       await post('/api/sensors/$id/calibration/start',