feat(plugin): PluginRelation 级联删除声明 + OnDeleteStrategy

新增 OnDeleteStrategy 枚举（Nullify/Cascade/Restrict）和
PluginRelation 结构体声明实体关联关系。PluginEntity 增加
relations 字段（serde(default) 向后兼容）。

crates/erp-plugin/src/dynamic_table.rs

@@ -956,6 +956,7 @@ mod tests {
                 },
             ],
             indexes: vec![],
+            relations: vec![],
         };
 
         let sql = DynamicTableManager::build_create_table_sql("erp_crm", &entity);
@@ -996,6 +997,7 @@ mod tests {
                 ..PluginField::default_for_field()
             }],
             indexes: vec![],
+            relations: vec![],
         };
 
         let sql = DynamicTableManager::build_create_table_sql("erp_crm", &entity);