routes: fix mypy no-any-return in get_pow_secret

app/api/routes.py

@@ -752,7 +752,8 @@ def is_admin() -> bool:
 
 def get_pow_secret() -> bytes:
     """Get PoW signing secret from app config."""
-    return current_app.config["POW_SECRET"].encode()
+    secret: str = current_app.config["POW_SECRET"]
+    return secret.encode()
 
 
 def generate_challenge(difficulty_override: int | None = None) -> dict[str, Any]: