feat: 添加 Goal, GoalRelation, StudentGoal 三个数据模型

- Goal: 目标表，支持存储学习目标
- GoalRelation: 目标自关联多对多表，支持 DAG 结构
- StudentGoal: 学员目标记录表，关联学员和目标

app/models.py

@@ -190,6 +190,66 @@ class StudentProblem(db.Model):
         }
 
 
+class Goal(db.Model):
+    """目标表"""
+    __tablename__ = "goals"
+
+    id = db.Column(db.Integer, primary_key=True)
+    name = db.Column(db.String(100), nullable=False)
+    content = db.Column(db.Text)
+    created_at = db.Column(db.DateTime, default=datetime.now)
+    updated_at = db.Column(db.DateTime, default=datetime.now, onupdate=datetime.now)
+
+    def to_dict(self):
+        return {
+            "id": self.id,
+            "name": self.name,
+            "content": self.content,
+            "created_at": self.created_at.isoformat() if self.created_at else None,
+            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
+        }
+
+
+class GoalRelation(db.Model):
+    """目标关联表 - 自关联多对多"""
+    __tablename__ = "goal_relations"
+
+    parent_goal_id = db.Column(db.Integer, db.ForeignKey("goals.id"), primary_key=True)
+    child_goal_id = db.Column(db.Integer, db.ForeignKey("goals.id"), primary_key=True)
+
+    parent = db.relationship("Goal", foreign_keys=[parent_goal_id], backref="child_relations")
+    child = db.relationship("Goal", foreign_keys=[child_goal_id], backref="parent_relations")
+
+
+class StudentGoal(db.Model):
+    """学员目标记录表"""
+    __tablename__ = "student_goals"
+
+    id = db.Column(db.Integer, primary_key=True)
+    student_id = db.Column(db.Integer, db.ForeignKey("students.id"), nullable=False)
+    goal_id = db.Column(db.Integer, db.ForeignKey("goals.id"), nullable=False)
+    status = db.Column(db.String(20), default="未开始")  # 未开始/进行中/已完成
+    mastery_level = db.Column(db.Integer, default=1)  # 1-5
+    deadline = db.Column(db.DateTime)
+    completed_at = db.Column(db.DateTime)
+    created_at = db.Column(db.DateTime, default=datetime.now)
+
+    student = db.relationship("Student", backref="goal_records")
+    goal = db.relationship("Goal")
+
+    def to_dict(self):
+        return {
+            "id": self.id,
+            "student_id": self.student_id,
+            "goal_id": self.goal_id,
+            "goal_name": self.goal.name if self.goal else None,
+            "status": self.status,
+            "mastery_level": self.mastery_level,
+            "deadline": self.deadline.isoformat() if self.deadline else None,
+            "completed_at": self.completed_at.isoformat() if self.completed_at else None,
+        }
+
+
 class PracticePlan(db.Model):
     """练习方案表"""