feat: 实现成长路径功能

- 新增数据库表: growth_path_nodes, user_growth_path_progress, user_node_completions
- 新增 Model: GrowthPathNode, UserGrowthPathProgress, UserNodeCompletion
- 新增 Service: GrowthPathService（管理端CRUD、学员端进度追踪）
- 新增 API: 学员端获取成长路径、管理端CRUD
- 前端学员端从API动态加载成长路径数据
- 更新管理端API接口定义

backend/app/models/__init__.py

@@ -37,6 +37,13 @@ from app.models.user_course_progress import (
     UserMaterialProgress,
     ProgressStatus,
 )
+from app.models.growth_path import (
+    GrowthPathNode,
+    UserGrowthPathProgress,
+    UserNodeCompletion,
+    GrowthPathStatus,
+    NodeStatus,
+)
 
 __all__ = [
     "Base",
@@ -80,4 +87,9 @@ __all__ = [
     "UserCourseProgress",
     "UserMaterialProgress",
     "ProgressStatus",
+    "GrowthPathNode",
+    "UserGrowthPathProgress",
+    "UserNodeCompletion",
+    "GrowthPathStatus",
+    "NodeStatus",
 ]