Function analyze 

pub async fn analyze(
    image_b64: &str,
    prompt: &str,
    provider: &LlmProvider,
    api_key: &str,
) -> Result<LlmOutput>

Analyze an image with a named LLM provider (Claude or Gemini).

image_b64 must be a base64-encoded JPEG (from compress::compress_for_llm).