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-app/src/main/java/com/cameleer3/server/app/controller/SearchController.java

@@ -172,6 +172,12 @@ public class SearchController {
         return ResponseEntity.ok(searchService.punchcard(from, to, application));
     }
 
+    @GetMapping("/attributes/keys")
+    @Operation(summary = "Distinct attribute key names across all executions")
+    public ResponseEntity<List<String>> attributeKeys() {
+        return ResponseEntity.ok(searchService.distinctAttributeKeys());
+    }
+
     @GetMapping("/errors/top")
     @Operation(summary = "Top N errors with velocity trend")
     public ResponseEntity<List<TopError>> topErrors(

cameleer3-server-app/src/main/java/com/cameleer3/server/app/search/ClickHouseSearchIndex.java

@@ -305,6 +305,21 @@ public class ClickHouseSearchIndex implements SearchIndex {
                    .replace("_", "\\_");
     }
 
+    @Override
+    public List<String> distinctAttributeKeys() {
+        try {
+            return jdbc.queryForList("""
+                    SELECT DISTINCT arrayJoin(JSONExtractKeys(attributes)) AS attr_key
+                    FROM executions FINAL
+                    WHERE tenant_id = 'default' AND attributes != '' AND attributes != '{}'
+                    ORDER BY attr_key
+                    """, String.class);
+        } catch (Exception e) {
+            log.error("Failed to query distinct attribute keys", e);
+            return List.of();
+        }
+    }
+
     private static Map<String, String> parseAttributesJson(String json) {
         if (json == null || json.isBlank()) return null;
         try {