Phase 2 scaffold: mitra online status & pairing logic

Add mitra online/offline status with heartbeat-based auto-offline,
customer-mitra pairing via Valkey pub/sub blast, session management,
and control center dashboard with real-time stats.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client_app/lib/core/api/api_client.dart

@@ -2,7 +2,7 @@ import 'package:dio/dio.dart';
 import 'package:firebase_auth/firebase_auth.dart';
 
 class ApiClient {
-  static const String _baseUrl = String.fromEnvironment(
+  static const String baseUrl = String.fromEnvironment(
     'API_BASE_URL',
     defaultValue: 'https://api.halobestie.com',
   );
@@ -10,7 +10,7 @@ class ApiClient {
   late final Dio _dio;
 
   ApiClient() {
-    _dio = Dio(BaseOptions(baseUrl: _baseUrl));
+    _dio = Dio(BaseOptions(baseUrl: baseUrl));
     _dio.interceptors.add(InterceptorsWrapper(
       onRequest: (options, handler) async {
         final user = FirebaseAuth.instance.currentUser;