加速错题 OCR：上传压缩、识别缩放、启动预热与 MKL-DNN。

backend/app/main.py

@@ -9,6 +9,7 @@ from fastapi.staticfiles import StaticFiles
 from app.core.config import settings
 from app.core.database import Base, SessionLocal, engine
 from app.routers import admin, auth, exams, export, settings as settings_router, students, subjects, wrong_questions
+from app.services import ocr as ocr_service
 from app.services.migrate import run_migrations
 from app.services.seed import seed_admin_and_settings, seed_subjects
 
@@ -34,6 +35,7 @@ async def lifespan(app: FastAPI):
         seed_admin_and_settings(db)
     finally:
         db.close()
+    ocr_service.warmup_ocr_engine()
     yield