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/questions.py

@@ -133,7 +133,7 @@ async def create_variant(question_id: str, user_id: str = Depends(get_current_us
         raise HTTPException(status_code=404, detail="Question not found")
 
     question = result.data[0]
-    variant_data = await asyncio.to_thread(generate_variant, question)
+    variant_data = await generate_variant(question)
     variant_data["knowledge_reminder"] = question.get("knowledge_reminder", "")
 
     saved = sb.table("question_variants").insert({