Get Library Prayer

Fetch a pre-generated prayer from the library for a given topic.

The language and voice are resolved from the user's profile preferences. Defaults to English ("en") and "v1-openai-alloy" if not set.

Args: topic: One of the 12 supported prayer topics (case-sensitive). current_user: Authenticated user (provides preferred_language and preferred_voice).

Returns: PrayerLibraryResponse with prayer text and audio URL.