feat: SQL 提交前自动格式化（sqlparse）

- format_code 新增 sql 分支，reindent + 关键字大写
- FormatCodeSerializer 的 language choices 加入 sql

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

submission/serializers.py

@@ -123,4 +123,4 @@ class SubmissionListSerializer(serializers.ModelSerializer):
 
 class FormatCodeSerializer(serializers.Serializer):
     code = serializers.CharField(max_length=1024 * 1024)
-    language = serializers.ChoiceField(choices=("python", "c", "cpp"))
+    language = serializers.ChoiceField(choices=("python", "c", "cpp", "sql"))