移除 OI 残留：UserProfile 的 oi_problems_status / total_score / add_score

判题不再写入 OI 相关状态后，UserProfile 上的 OI 字段永远为空/为 0：
- 删除 oi_problems_status、total_score 两列及 add_score() 方法
- ProfileProblemDisplayIDRefreshAPI 去掉 oi_problems 合并分支

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

account/migrations/0008_remove_userprofile_oi_problems_status_and_more.py

@@ -0,0 +1,21 @@
+# Generated by Django 6.0.4 on 2026-07-04 17:32
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('account', '0007_rename_admin_to_student_admin'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='userprofile',
+            name='oi_problems_status',
+        ),
+        migrations.RemoveField(
+            model_name='userprofile',
+            name='total_score',
+        ),
+    ]