P2.1: Add GatewayConnector BackgroundService with WebSocket client

- GatewayConnector: BackgroundService maintaining persistent WebSocket connection
  to OpenClaw Gateway with Bearer token auth from backend env only
- IGatewayConnector: Interface exposing connection state, version info, health
- Exponential backoff reconnect: 1s initial, max 5min, with jitter
- Version pinning: RequiredVersion from config; fail-fast or warn on mismatch
- Pre-flight HTTP version check before WebSocket connect
- GatewayHealthController: GET /api/health/gateway with connection status
- GatewayConnectorOptions: Configurable via GatewayConnector config section
- Token never logged; read from Integrations:OpenClaw:Password/Token only
- 16 tests: options defaults, config binding, health endpoint responses,
  connection states, DI registration, routing attributes

backend/Extensions/ServiceCollectionExtensions.cs

@@ -229,6 +229,12 @@ public static class ServiceCollectionExtensions
         services.AddScoped<IStaleTaskRecoveryService, StaleTaskRecoveryService>();
         services.AddHostedService<StaleTaskRecoveryBackgroundService>();
 
+        // ── Gateway WebSocket Connector ──
+        services.AddOptions<GatewayConnectorOptions>()
+            .BindConfiguration(GatewayConnectorOptions.SectionName);
+        services.AddSingleton<IGatewayConnector, GatewayConnector>();
+        services.AddHostedService(sp => (GatewayConnector)sp.GetRequiredService<IGatewayConnector>());
+
         // ── Backend Bridge (Agent-Command-Service) ──
         services.AddScoped<ITaskBridgeService, TaskBridgeService>();