[OpenAI] Add optional AI-assisted study drafting in the private admin app

[troyscott]

Outcome

Use the OpenAI Responses API to create unpublished candidate explanations, recall questions, and change summaries from approved source material.

Acceptance criteria

• AsyncOpenAI is created and closed through FastAPI lifespan.
• API key and model configuration remain server-side and out of Git.
• The public reader exposes no generic prompt endpoint.
• Output is always a draft requiring human review.
• Prompts require source-grounded output and preserve exact source references.
• The MVP remains usable when OpenAI is not configured.