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/Services/IGatewayConnector.cs

@@ -0,0 +1,57 @@
+using Nexus.Api.Models;
+
+namespace Nexus.Api.Services;
+
+/// <summary>
+/// Maintains a persistent WebSocket connection to the OpenClaw Gateway
+/// for real-time event streaming and health monitoring.
+/// </summary>
+public interface IGatewayConnector
+{
+    /// <summary>
+    /// Current WebSocket connection state.
+    /// </summary>
+    GatewayConnectionState ConnectionState { get; }
+
+    /// <summary>
+    /// Gateway version reported at last connection.
+    /// </summary>
+    string? GatewayVersion { get; }
+
+    /// <summary>
+    /// Required version from configuration, or null when unpinned.
+    /// </summary>
+    string? RequiredVersion { get; }
+
+    /// <summary>
+    /// Timestamp of the last successful connection attempt.
+    /// </summary>
+    DateTimeOffset? LastConnectedAt { get; }
+
+    /// <summary>
+    /// Number of consecutive failed connection attempts.
+    /// </summary>
+    int ReconnectAttempts { get; }
+
+    /// <summary>
+    /// Detailed status message (e.g. error or version info).
+    /// </summary>
+    string? StatusMessage { get; }
+}
+
+/// <summary>
+/// Represents the current connection state of the GatewayConnector.
+/// </summary>
+public enum GatewayConnectionState
+{
+    /// <summary>Not yet attempted or initializing.</summary>
+    Initializing,
+    /// <summary>Connected and healthy.</summary>
+    Connected,
+    /// <summary>Disconnected, waiting for reconnect.</summary>
+    Disconnected,
+    /// <summary>Recoverable error, retrying.</summary>
+    Reconnecting,
+    /// <summary>Failed after maximum retries.</summary>
+    Failed
+}