feat: expandable previews, KaTeX rendering, variant speedup, batch import

- Analytics/Similar: expandable question preview with KaTeX rendering
- KaTeXRenderer: auto markdown-to-HTML (code blocks, tables, bold), auto Unicode→LaTeX
- ErrorBook: full question text rendering instead of truncated preview
- Variant: remove hint/solution from generation (faster), async, fix null crash
- Grading: add max_tokens limit
- JSON parser: robust multi-layer repair + JSONDecodeError retry
- Extraction prompt: enforce LaTeX notation for math
- Upload: redirect to home instead of blank paper page
- ProcessingBanner: add ETA time estimate + percentage
- Batch import script + handoff guide for team

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/analytics.py

@@ -230,6 +230,7 @@ async def get_course_analytics(course_code: str):
             "source": source_label,
             "question_number": question["question_number"],
             "preview": question["question_text"][:220],
+            "full_text": question["question_text"],
             "difficulty": question.get("difficulty"),
             "question_type": question_type,
             "year": paper.get("year"),