feat(plugin): 评估量表插件骨架 — assessment_scale + assessment_response + PHQ-9 默认数据

- 创建 erp-plugin-assessment cdylib crate
- 实现 Guest trait（init/on_tenant_created/handle_event）
- on_tenant_created 自动插入 PHQ-9 抑郁筛查量表
- plugin.toml 声明 2 实体 + 4 权限 + 触发事件

Cargo.toml

@@ -17,6 +17,7 @@ members = [
     "crates/erp-plugin-itops",
     "crates/erp-health",
     "crates/erp-ai",
+    "crates/erp-plugin-assessment",
 ]
 
 [workspace.package]