Add flag field to Submission model with FlagChoices enum

Adds red/blue/green/yellow flag options for marking submissions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

submission/migrations/0004_submission_flag.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.1 on 2026-03-09 09:22
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submission', '0003_submission_conversation'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='submission',
+            name='flag',
+            field=models.CharField(blank=True, choices=[('red', '值得展示'), ('blue', '需要讲解'), ('green', '优秀作品'), ('yellow', '需要改进')], default=None, max_length=10, null=True, verbose_name='标记'),
+        ),
+    ]