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>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/SearchService.java

@@ -25,6 +25,10 @@ public class SearchService {
         return searchIndex.count(request);
     }
 
+    public List<String> distinctAttributeKeys() {
+        return searchIndex.distinctAttributeKeys();
+    }
+
     public ExecutionStats stats(Instant from, Instant to) {
         return statsStore.stats(from, to);
     }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/storage/SearchIndex.java

@@ -5,6 +5,8 @@ import com.cameleer3.server.core.search.SearchRequest;
 import com.cameleer3.server.core.search.SearchResult;
 import com.cameleer3.server.core.storage.model.ExecutionDocument;
 
+import java.util.List;
+
 public interface SearchIndex {
 
     SearchResult<ExecutionSummary> search(SearchRequest request);
@@ -14,4 +16,7 @@ public interface SearchIndex {
     void index(ExecutionDocument document);
 
     void delete(String executionId);
+
+    /** Returns distinct attribute key names across all executions. */
+    List<String> distinctAttributeKeys();
 }