新增作文区与 AI 解读开关，修复 CSV 导出。

系统设置可关闭成绩复盘 AI；学生详情增加作文区（OCR/手动题目、方案与范文、历史与 MD 下载）；导出改用 UTF-8 文件名响应。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/main.py

@@ -8,7 +8,7 @@ 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.routers import admin, auth, compositions, 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
@@ -57,6 +57,7 @@ app.include_router(students.router, prefix="/api")
 app.include_router(subjects.router, prefix="/api")
 app.include_router(exams.router, prefix="/api")
 app.include_router(wrong_questions.router, prefix="/api")
+app.include_router(compositions.router, prefix="/api")
 app.include_router(export.router, prefix="/api")