Fixing mypy and ruff errors.

src/app.py

@@ -1,16 +1,17 @@
-from flask import Flask, jsonify
-from datetime import datetime, timezone
+from datetime import UTC, datetime
+
+from flask import Flask, Response, jsonify
 
 app = Flask(__name__)
 
 
 @app.get("/heartbeat")
-def heartbeat() -> tuple:
+def heartbeat() -> tuple[Response, int]:
     return (
         jsonify(
             {
                 "status": "ok",
-                "timestamp": datetime.now(timezone.utc).isoformat(),
+                "timestamp": datetime.now(UTC).isoformat(),
             }
         ),
         200,