feat: add route_fallbacks metric to Prometheus and Zabbix monitoring

- Prometheus: new pulse_route_fallbacks_total counter
- Zabbix: extend response with route_requests_total, route_fallbacks_total, uptime_seconds
- Docs: update Prometheus metrics table and Zabbix response example

docs/api.md

@@ -216,7 +216,10 @@ Use `X-API-Key` header (not JWT).
       "{#IS_STALE}": false,
       "{#SECONDS_AGO}": 3.0
     }
-  ]
+  ],
+  "route_requests_total": 5000,
+  "route_fallbacks_total": 42,
+  "uptime_seconds": 7200
 }
 ```
 
@@ -225,6 +228,7 @@ Use `X-API-Key` header (not JWT).
 | Метрика | Type | Labels |
 |---------|------|--------|
 | `pulse_route_requests_total` | counter | – |
+| `pulse_route_fallbacks_total` | counter | – |
 | `pulse_nodes_total` | gauge | – |
 | `pulse_nodes_healthy` | gauge | – |
 | `pulse_uptime_seconds` | gauge | – |