feat: 完善成长路径画布设计器

后端:
- 添加 position_x, position_y 字段保存节点位置

前端:
- 支持从节点右侧圆点拖拽出箭头连接到其他课程
- 自动根据节点Y坐标识别所属阶段
- 保存并恢复节点位置，不再重置
- 阶段区域高亮显示
- 循环依赖检测

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/growth_path.py

@@ -86,6 +86,14 @@ class GrowthPathNode(BaseModel, SoftDeleteMixin):
     estimated_days: Mapped[int] = mapped_column(
         Integer, default=7, nullable=False, comment="预计学习天数"
     )
+    
+    # 画布位置（用于可视化编辑器）
+    position_x: Mapped[Optional[int]] = mapped_column(
+        Integer, nullable=True, default=0, comment="画布X坐标"
+    )
+    position_y: Mapped[Optional[int]] = mapped_column(
+        Integer, nullable=True, default=0, comment="画布Y坐标"
+    )
 
     # 关联关系
     growth_path: Mapped["GrowthPath"] = relationship(  # noqa: F821

backend/app/schemas/growth_path.py

@@ -28,6 +28,8 @@ class NodeBase(BaseModel):
     is_required: bool = Field(True, description="是否必修")
     prerequisites: Optional[List[int]] = Field(None, description="前置节点IDs")
     estimated_days: int = Field(7, description="预计学习天数")
+    position_x: Optional[int] = Field(0, description="画布X坐标")
+    position_y: Optional[int] = Field(0, description="画布Y坐标")
 
 
 # =====================================================