refactor(search): drop dead SearchIndexer subsystem

After the ExecutionController removal (0f635576), SearchIndexer
subscribed to ExecutionUpdatedEvent but nothing publishes that event.
Every SearchIndexerStats metric returned always-zero, and the admin
/api/v1/admin/clickhouse/pipeline endpoint that surfaced those stats
carried no signal.

Backend removed:
- core: SearchIndexer, SearchIndexerStats, ExecutionUpdatedEvent
- app: IndexerPipelineResponse DTO, /pipeline endpoint on
  ClickHouseAdminController (field + ctor param)
- StorageBeanConfig.searchIndexer bean

UI removed:
- IndexerPipeline type + useIndexerPipeline hook in
  api/queries/admin/clickhouse.ts
- Indexer Pipeline card in ClickHouseAdminPage.tsx (plus ProgressBar
  import and pipeline* CSS classes)

OpenAPI schema.d.ts + openapi.json regenerated (stale /pipeline path
and IndexerPipelineResponse schema removed).

SearchIndex interface + ClickHouseSearchIndex impl kept — those are
live and used by SearchService + ExchangeMatchEvaluator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/api/schema.d.ts

@@ -2044,23 +2044,6 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
-    "/admin/clickhouse/pipeline": {
-        parameters: {
-            query?: never;
-            header?: never;
-            path?: never;
-            cookie?: never;
-        };
-        /** Search indexer pipeline statistics */
-        get: operations["getPipeline"];
-        put?: never;
-        post?: never;
-        delete?: never;
-        options?: never;
-        head?: never;
-        patch?: never;
-        trace?: never;
-    };
     "/admin/clickhouse/performance": {
         parameters: {
             query?: never;
@@ -3633,23 +3616,6 @@ export interface components {
             readRows?: number;
             query?: string;
         };
-        /** @description Search indexer pipeline statistics */
-        IndexerPipelineResponse: {
-            /** Format: int32 */
-            queueDepth?: number;
-            /** Format: int32 */
-            maxQueueSize?: number;
-            /** Format: int64 */
-            failedCount?: number;
-            /** Format: int64 */
-            indexedCount?: number;
-            /** Format: int64 */
-            debounceMs?: number;
-            /** Format: double */
-            indexingRate?: number;
-            /** Format: date-time */
-            lastIndexedAt?: string;
-        };
         /** @description ClickHouse storage and performance metrics */
         ClickHousePerformanceResponse: {
             diskSize?: string;
@@ -7942,26 +7908,6 @@ export interface operations {
             };
         };
     };
-    getPipeline: {
-        parameters: {
-            query?: never;
-            header?: never;
-            path?: never;
-            cookie?: never;
-        };
-        requestBody?: never;
-        responses: {
-            /** @description OK */
-            200: {
-                headers: {
-                    [name: string]: unknown;
-                };
-                content: {
-                    "*/*": components["schemas"]["IndexerPipelineResponse"];
-                };
-            };
-        };
-    };
     getPerformance: {
         parameters: {
             query?: never;