Verify Oura Webhook

Respond to Oura's subscription verification challenge.

When creating a webhook subscription, Oura sends a GET request with verification_token and challenge query params. We must confirm the token matches our configured secret and echo back the challenge.

Args: verification_token: Token Oura received when the subscription was created. challenge: Random string Oura expects in the response body.

Returns: JSON body containing the challenge string.

Raises: HTTPException: If the verification token does not match.