feat: 成长路径支持多岗位关联 + 增强拖拽功能

前端:
- 岗位选择改为多选模式
- 增强拖拽视觉反馈（高亮、动画提示）
- 列表显示多个岗位标签

后端:
- 添加 position_ids 字段支持多岗位
- 兼容旧版 position_id 单选数据
- 返回 position_names 数组

backend/app/schemas/growth_path.py

@@ -56,7 +56,8 @@ class GrowthPathCreate(BaseModel):
     name: str = Field(..., description="路径名称")
     description: Optional[str] = Field(None, description="路径描述")
     target_role: Optional[str] = Field(None, description="目标角色")
-    position_id: Optional[int] = Field(None, description="关联岗位ID")
+    position_id: Optional[int] = Field(None, description="关联岗位ID（兼容旧版）")
+    position_ids: Optional[List[int]] = Field(None, description="关联岗位ID列表（支持多选）")
     stages: Optional[List[StageConfig]] = Field(None, description="阶段配置")
     estimated_duration_days: Optional[int] = Field(None, description="预计完成天数")
     is_active: bool = Field(True, description="是否启用")
@@ -70,6 +71,7 @@ class GrowthPathUpdate(BaseModel):
     description: Optional[str] = None
     target_role: Optional[str] = None
     position_id: Optional[int] = None
+    position_ids: Optional[List[int]] = None
     stages: Optional[List[StageConfig]] = None
     estimated_duration_days: Optional[int] = None
     is_active: Optional[bool] = None