feat(reaction): 新增 reaction app 与数据模型

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

reaction/models.py

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+from django.db import models
+
+from account.models import User
+from problem.models import Problem
+
+
+class ReactionType(models.TextChoices):
+    TOO_EASY = "too_easy", "太简单"
+    TOO_HARD = "too_hard", "太难了"
+    CONFUSING = "confusing", "没看懂"
+    BUGGY = "buggy", "题目有错"
+    LEARNED = "learned", "学到了"
+    INTERESTING = "interesting", "有意思"
+    WANT_EXPLAIN = "want_explain", "想听讲解"
+
+
+class Reaction(models.Model):
+    problem = models.ForeignKey(Problem, on_delete=models.CASCADE)
+    user = models.ForeignKey(User, on_delete=models.CASCADE)
+    type = models.CharField(max_length=20, choices=ReactionType.choices, verbose_name="表情类型")
+    create_time = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        db_table = "reaction"
+        unique_together = ("problem", "user", "type")
+        indexes = [
+            models.Index(fields=["problem", "type"], name="reaction_problem_type_idx"),
+        ]