refactor(blockchain-explorer): add HTTPException import for error handling

- Import HTTPException from fastapi for proper exception handling in endpoints
- Prepare for enhanced error responses in blockchain explorer API routes

apps/blockchain-explorer/main.py

@@ -9,7 +9,7 @@ import httpx
 import json
 from datetime import datetime
 from typing import Dict, List, Optional, Any
-from fastapi import FastAPI, Request
+from fastapi import FastAPI, Request, HTTPException
 from fastapi.responses import HTMLResponse
 from fastapi.staticfiles import StaticFiles
 import uvicorn