feat(plugin): 新增数据统计 REST API — count 和 aggregate 端点

- dynamic_table: 新增 build_filtered_count_sql（带过滤/搜索的 COUNT）和 build_aggregate_sql（按字段分组计数）
- data_service: 新增 count 和 aggregate 方法，支持实时统计查询
- data_handler: 新增 count_plugin_data 和 aggregate_plugin_data REST handler
- data_dto: 新增 AggregateItem、AggregateQueryParams、CountQueryParams 类型
- module: 注册 /plugins/{plugin_id}/{entity}/count 和 /aggregate 路由
- 包含 8 个新增单元测试，全部通过

crates/erp-plugin/src/data_service.rs

@@ -261,6 +261,99 @@ impl PluginDataService {
 
         Ok(())
     }
+
+    /// 统计记录数（支持过滤和搜索）
+    pub async fn count(
+        plugin_id: Uuid,
+        entity_name: &str,
+        tenant_id: Uuid,
+        db: &sea_orm::DatabaseConnection,
+        filter: Option<serde_json::Value>,
+        search: Option<String>,
+    ) -> AppResult<u64> {
+        let table_name = resolve_table_name(plugin_id, entity_name, tenant_id, db).await?;
+
+        // 获取 searchable 字段列表，构建搜索条件
+        let entity_fields = resolve_entity_fields(plugin_id, entity_name, tenant_id, db).await?;
+        let search_tuple = {
+            let searchable: Vec<&str> = entity_fields
+                .iter()
+                .filter(|f| f.searchable == Some(true))
+                .map(|f| f.name.as_str())
+                .collect();
+            match (searchable.is_empty(), &search) {
+                (false, Some(kw)) => Some((searchable.join(","), kw.clone())),
+                _ => None,
+            }
+        };
+
+        let (sql, values) = DynamicTableManager::build_filtered_count_sql(
+            &table_name,
+            tenant_id,
+            filter,
+            search_tuple,
+        )
+        .map_err(|e| AppError::Validation(e))?;
+
+        #[derive(FromQueryResult)]
+        struct CountResult {
+            count: i64,
+        }
+
+        let result = CountResult::find_by_statement(Statement::from_sql_and_values(
+            sea_orm::DatabaseBackend::Postgres,
+            sql,
+            values,
+        ))
+        .one(db)
+        .await?
+        .map(|r| r.count as u64)
+        .unwrap_or(0);
+
+        Ok(result)
+    }
+
+    /// 聚合查询 — 按字段分组计数
+    /// 返回 [(分组键, 计数), ...]
+    pub async fn aggregate(
+        plugin_id: Uuid,
+        entity_name: &str,
+        tenant_id: Uuid,
+        db: &sea_orm::DatabaseConnection,
+        group_by_field: &str,
+        filter: Option<serde_json::Value>,
+    ) -> AppResult<Vec<(String, i64)>> {
+        let table_name = resolve_table_name(plugin_id, entity_name, tenant_id, db).await?;
+
+        let (sql, values) = DynamicTableManager::build_aggregate_sql(
+            &table_name,
+            tenant_id,
+            group_by_field,
+            filter,
+        )
+        .map_err(|e| AppError::Validation(e))?;
+
+        #[derive(FromQueryResult)]
+        struct AggRow {
+            key: Option<String>,
+            count: i64,
+        }
+
+        let rows = AggRow::find_by_statement(Statement::from_sql_and_values(
+            sea_orm::DatabaseBackend::Postgres,
+            sql,
+            values,
+        ))
+        .all(db)
+        .await?;
+
+        let result = rows
+            .into_iter()
+            .map(|r| (r.key.unwrap_or_default(), r.count))
+            .collect();
+
+        Ok(result)
+    }
 }
 
 /// 从 plugin_entities 表解析 table_name（带租户隔离）