feat(reaction): 新增 reaction app 与数据模型

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

reaction/serializers.py

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+from reaction.models import ReactionType
+from utils.api import serializers
+
+MAX_REACTIONS = 3
+
+
+class SetReactionSerializer(serializers.Serializer):
+    problem_id = serializers.IntegerField()
+    types = serializers.ListField(
+        child=serializers.ChoiceField(choices=ReactionType.choices),
+        allow_empty=True,
+    )
+
+    def validate_types(self, value):
+        unique = list(dict.fromkeys(value))
+        if len(unique) > MAX_REACTIONS:
+            raise serializers.ValidationError(f"最多只能选 {MAX_REACTIONS} 个")
+        return unique