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>