feat: add correlation chain and processor count to Exchange Detail

Adds a recursive processor count stat to the exchange header, and a
Correlation Chain section that visualises related executions sharing
the same correlationId, with the current exchange highlighted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/api/queries/correlation.ts

@@ -0,0 +1,20 @@
+import { useQuery } from '@tanstack/react-query';
+import { api } from '../client';
+
+export function useCorrelationChain(correlationId: string | null) {
+  return useQuery({
+    queryKey: ['correlation-chain', correlationId],
+    queryFn: async () => {
+      const { data } = await api.POST('/search/executions', {
+        body: {
+          correlationId: correlationId!,
+          limit: 20,
+          sortField: 'startTime',
+          sortDir: 'asc',
+        },
+      });
+      return data;
+    },
+    enabled: !!correlationId,
+  });
+}