feat: add Clear History button to delete all consumed instructions

- Backend: instruction_service.clear_consumed() bulk-deletes consumed rows
- Backend: DELETE /api/instructions/consumed route (preserves pending)
- Frontend: Clear button in consumed panel header (hidden when empty)
- Frontend: SSE handler for history.cleared event - instant UI update
- Frontend: api.clearConsumed() fetch wrapper

static/js/events.js

@@ -104,6 +104,11 @@ function _handleEvent(event) {
       }
       break;
     }
+    case 'history.cleared': {
+      const next = (state.get('instructions') || []).filter(i => i.status !== 'consumed');
+      state.set('instructions', next);
+      break;
+    }
     case 'status.changed': {
       api.status().then(s => state.set('status', s)).catch(console.error);
       break;