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/services/grader.py

@@ -68,9 +68,7 @@ Return JSON:
   "question_text": "HTML formatted variant question",
   "question_type": "{question_type}",
   "options": [MC only, format {{"label":"A","text":"..."}}, ...] or null,
-  "correct_answer": "Correct answer (plain text)",
-  "ai_hint": "HTML formatted hint that guides thinking WITHOUT giving the answer",
-  "solution": "HTML formatted complete step-by-step solution"
+  "correct_answer": "Correct answer (plain text)"
 }}"""
 
 
@@ -90,7 +88,7 @@ def ocr_photo(photo_bytes: bytes) -> str:
             ]},
         ],
         temperature=0,
-        max_tokens=2000,
+        max_tokens=1500,
     )
     return resp.choices[0].message.content or ""
 
@@ -114,13 +112,15 @@ def grade_answer(question: dict, student_answer: str) -> dict:
             )},
         ],
         temperature=0.2,
+        max_tokens=2048,
         response_format={"type": "json_object"},
     )
     return json.loads(resp.choices[0].message.content)
 
 
-def generate_variant(question: dict) -> dict:
-    """Gemini generates a variant question"""
+async def generate_variant(question: dict) -> dict:
+    """DeepSeek generates a variant question (async)"""
+    import asyncio
     answer = (
         question.get("correct_option")
         or question.get("correct_answer")
@@ -129,18 +129,20 @@ def generate_variant(question: dict) -> dict:
     )
 
     ds = get_deepseek_client()
-    resp = ds.chat.completions.create(
+    prompt = VARIANT_PROMPT.format(
+        question_type=question["question_type"],
+        question_text=question["question_text"],
+        topics=", ".join(question.get("topics", [])),
+        difficulty=question.get("difficulty", "medium"),
+        answer=answer,
+    )
+
+    resp = await asyncio.to_thread(
+        ds.chat.completions.create,
         model="deepseek-chat",
-        messages=[
-            {"role": "system", "content": VARIANT_PROMPT.format(
-                question_type=question["question_type"],
-                question_text=question["question_text"],
-                topics=", ".join(question.get("topics", [])),
-                difficulty=question.get("difficulty", "medium"),
-                answer=answer,
-            )},
-        ],
+        messages=[{"role": "system", "content": prompt}],
         temperature=0.5,
+        max_tokens=2048,
         response_format={"type": "json_object"},
     )
     return json.loads(resp.choices[0].message.content)