fix: add missing HTMLResponse import to blockchain-explorer main.py

The blockchain-explorer was failing with NameError because it used HTMLResponse
but did not import it from fastapi.responses.

apps/blockchain-explorer/main.py

@@ -14,7 +14,7 @@ from typing import Optional, List, Dict, Any
 import os
 
 from fastapi import FastAPI, HTTPException, Query
-from fastapi.responses import JSONResponse
+from fastapi.responses import JSONResponse, HTMLResponse
 from fastapi.middleware.cors import CORSMiddleware
 from pydantic import BaseModel, Field
 import uvicorn