feat(plugin): P1 跨插件数据引用系统 — 后端 Phase 1-3

实现跨插件实体引用的基础后端能力：

Phase 1 — Manifest 扩展 + Entity Registry 数据层:
- PluginField 新增 ref_plugin/ref_fallback_label 支持跨插件引用声明
- PluginRelation 新增 name/relation_type/display_field（CRM 已在用的字段）
- PluginEntity 新增 is_public 标记可被其他插件引用的实体
- 数据库迁移：plugin_entities 新增 manifest_id + is_public 列 + 索引
- SeaORM Entity 和 install 流程同步更新

Phase 2 — 后端跨插件引用解析 + 校验:
- data_service: 新增 resolve_cross_plugin_entity/is_plugin_active 函数
- validate_ref_entities: 支持 ref_plugin 字段，目标插件未安装时跳过校验（软警告）
- host.rs: HostState 新增 cross_plugin_entities 映射，db_query 支持点分记号
- engine.rs: execute_wasm 自动构建跨插件实体映射

Phase 3 — API 端点:
- POST /plugins/{id}/{entity}/resolve-labels 批量标签解析
- GET /plugin-registry/entities 公开实体注册表查询

crates/erp-plugin/src/data_dto.rs

@@ -55,6 +55,18 @@ pub struct AggregateItem {
     pub count: i64,
 }
 
+/// 多聚合查询响应项
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct AggregateMultiRow {
+    /// 分组键
+    pub key: String,
+    /// 计数
+    pub count: i64,
+    /// 聚合指标: {"sum_amount": 5000.0, "avg_price": 25.5}
+    #[serde(default)]
+    pub metrics: std::collections::HashMap<String, f64>,
+}
+
 /// 聚合查询参数
 #[derive(Debug, Serialize, Deserialize, utoipa::IntoParams)]
 pub struct AggregateQueryParams {
@@ -64,6 +76,26 @@ pub struct AggregateQueryParams {
     pub filter: Option<String>,
 }
 
+/// 多聚合查询请求体
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct AggregateMultiReq {
+    /// 分组字段名
+    pub group_by: String,
+    /// 聚合定义列表: [{"func": "sum", "field": "amount"}]
+    pub aggregations: Vec<AggregateDefDto>,
+    /// JSON 格式过滤
+    pub filter: Option<serde_json::Value>,
+}
+
+/// 单个聚合定义
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct AggregateDefDto {
+    /// 聚合函数: count, sum, avg, min, max
+    pub func: String,
+    /// 字段名
+    pub field: String,
+}
+
 /// 统计查询参数
 #[derive(Debug, Serialize, Deserialize, utoipa::IntoParams)]
 pub struct CountQueryParams {
@@ -105,3 +137,29 @@ pub struct TimeseriesItem {
     /// 计数
     pub count: i64,
 }
+
+// ─── 跨插件引用 DTO ──────────────────────────────────────────────────
+
+/// 批量标签解析请求
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct ResolveLabelsReq {
+    /// 字段名 → UUID 列表
+    pub fields: std::collections::HashMap<String, Vec<String>>,
+}
+
+/// 批量标签解析响应
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct ResolveLabelsResp {
+    /// 字段名 → { uuid: label } 映射
+    pub labels: serde_json::Value,
+    /// 字段名 → 目标插件元信息
+    pub meta: serde_json::Value,
+}
+
+/// 公开实体信息（实体注册表查询响应）
+#[derive(Debug, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct PublicEntityResp {
+    pub manifest_id: String,
+    pub entity_name: String,
+    pub display_name: String,
+}