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/routers/admin.py

@@ -26,6 +26,7 @@ router = APIRouter(prefix="/admin", tags=["admin"])
 def settings_to_out(row: SystemSettings) -> SystemSettingsOut:
     return SystemSettingsOut(
         registration_enabled=row.registration_enabled,
+        ai_review_enabled=getattr(row, "ai_review_enabled", True),
         ai_provider=AIProviderEnum(row.ai_provider or "ollama"),
         ollama_base_url=row.ollama_base_url,
         ollama_model=row.ollama_model,
@@ -64,6 +65,8 @@ def update_settings(
     row = get_or_create_settings(db)
     if data.registration_enabled is not None:
         row.registration_enabled = data.registration_enabled
+    if data.ai_review_enabled is not None:
+        row.ai_review_enabled = data.ai_review_enabled
     if data.ai_provider is not None:
         row.ai_provider = data.ai_provider.value
     if data.ollama_base_url is not None: