Add HLE attribution: per-question author/id credit + dataset citation

• Augment questions.json with author_name from cais/hle (build script too)
• Thread author through Question -> GradedQuestion -> submit response
• Results view: 'HLE #<id> · contributed by <author>' linking to the dataset
• Footer: 'questions from Humanity's Last Exam (arXiv:2501.14249, MIT)'
• README: data-source note + official Nature/arXiv BibTeX + how to cite a question

Verified all 158 questions are verbatim from cais/hle (15/15 exact-match sample, all ids present).