feat: pipeline on/off toggle with per-stage Helm control

- Added pipelineEnabled flag to Helm values (default: true)
- Worker services (scheduler, ingestion, parser, extractor, aggregation,
  recommendation, broker-adapter, lake-publisher) scale to 0 when disabled
- API services always run regardless of toggle
- Redis-based runtime toggle: POST /api/ops/pipeline/toggle
- Scheduler checks the flag before each cycle
- Frontend: green/red Pipeline ON/OFF button on the pipeline page
- Beta defaults to pipelineEnabled: false
- Base values.yaml: blanked external URLs (Ollama, Polygon, Alpaca)
  so stages only connect to what they explicitly configure

frontend/src/api/hooks.ts

@@ -533,6 +533,14 @@ export function useRetryFailedExtractions() {
   });
 }
 
+export function usePipelineToggle() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (enabled: boolean) => apiPost<{ pipeline_enabled: boolean }>('query', '/api/ops/pipeline/toggle', { enabled }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['pipeline-health'] }),
+  });
+}
+
 export function useIngestionSummary(hours = 24) {
   return useGet<Record<string, unknown>>(['ingestion-summary', hours], 'query', `/api/ops/ingestion/summary?hours=${hours}`);
 }