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/appsettings.json

@@ -27,5 +27,12 @@
     "StaleHours": 2,
     "IntervalMinutes": 30
   },
+  "GatewayConnector": {
+    "WebSocketPath": "/ws",
+    "ReconnectInitialDelayMs": 1000,
+    "ReconnectMaxDelayMs": 300000,
+    "FailFastOnVersionMismatch": true,
+    "FailFastOnMissingVersion": false
+  },
   "AllowedHosts": "*"
 }