feat: show distinct attribute keys in cmd-k Attributes tab

Add GET /search/attributes/keys endpoint that queries distinct
attribute key names from ClickHouse using JSONExtractKeys. Attribute
keys appear in the cmd-k Attributes tab alongside attribute value
matches from exchange results.

- SearchIndex.distinctAttributeKeys() interface method
- ClickHouseSearchIndex implementation using arrayJoin(JSONExtractKeys)
- SearchController /attributes/keys endpoint
- useAttributeKeys() React Query hook
- buildSearchData includes attribute keys as 'attribute' category items

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/queries/executions.ts

@@ -32,6 +32,22 @@ export function useExecutionStats(
   });
 }
 
+export function useAttributeKeys() {
+  return useQuery({
+    queryKey: ['search', 'attribute-keys'],
+    queryFn: async () => {
+      const token = (await import('../../auth/auth-store')).useAuthStore.getState().accessToken;
+      const { config } = await import('../../config');
+      const res = await fetch(`${config.apiBaseUrl}/search/attributes/keys`, {
+        headers: { Authorization: `Bearer ${token}` },
+      });
+      if (!res.ok) throw new Error('Failed to load attribute keys');
+      return res.json() as Promise<string[]>;
+    },
+    staleTime: 60_000,
+  });
+}
+
 export function useSearchExecutions(filters: SearchRequest, live = false) {
   const liveQuery = useLiveQuery(5_000);
   return useQuery({