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);

crates/erp-plugin/src/manifest.rs

@@ -43,6 +43,8 @@ pub struct PluginEntity {
     pub fields: Vec<PluginField>,
     #[serde(default)]
     pub indexes: Vec<PluginIndex>,
+    #[serde(default)]
+    pub relations: Vec<PluginRelation>,
 }
 
 /// 字段校验规则
@@ -156,6 +158,23 @@ pub struct PluginIndex {
     pub unique: bool,
 }
 
+/// 级联删除策略
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename_all = "snake_case")]
+pub enum OnDeleteStrategy {
+    Nullify,   // 置空外键字段
+    Cascade,   // 级联软删除
+    Restrict,  // 存在关联时拒绝删除
+}
+
+/// 实体关联关系声明
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PluginRelation {
+    pub entity: String,
+    pub foreign_key: String,
+    pub on_delete: OnDeleteStrategy,
+}
+
 /// 事件订阅配置
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct PluginEvents {
@@ -690,4 +709,41 @@ no_cycle = true
         assert_eq!(field.no_cycle, Some(true));
         assert_eq!(field.ref_entity.as_deref(), Some("customer"));
     }
+
+    #[test]
+    fn parse_entity_with_relations() {
+        let toml = r#"
+[metadata]
+id = "test"
+name = "Test"
+version = "0.1.0"
+
+[schema]
+[[schema.entities]]
+name = "customer"
+display_name = "客户"
+
+[[schema.entities.fields]]
+name = "code"
+field_type = "string"
+required = true
+display_name = "编码"
+
+[[schema.entities.relations]]
+entity = "contact"
+foreign_key = "customer_id"
+on_delete = "cascade"
+
+[[schema.entities.relations]]
+entity = "customer_tag"
+foreign_key = "customer_id"
+on_delete = "cascade"
+"#;
+        let manifest = parse_manifest(toml).unwrap();
+        let entity = &manifest.schema.unwrap().entities[0];
+        assert_eq!(entity.relations.len(), 2);
+        assert_eq!(entity.relations[0].entity, "contact");
+        assert_eq!(entity.relations[0].foreign_key, "customer_id");
+        assert!(matches!(entity.relations[0].on_delete, OnDeleteStrategy::Cascade));
+    }
 }