feat(oa): add oa_instance_comment table and model

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/sql/migrations/2026-09-20-oa-instance-comment.sql

@@ -0,0 +1,20 @@
+-- OA 审批实例评论
+CREATE TABLE IF NOT EXISTS `oa_instance_comment` (
+  `id` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '主键ID',
+  `instance_id` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '所属实例ID',
+  `content` varchar(1024) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '评论正文',
+  `attachment_refs_json` text COLLATE utf8mb4_unicode_ci COMMENT '引用表单附件快照 JSON',
+  `status` varchar(16) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT 'normal' COMMENT 'normal/revoked',
+  `revoked_time` datetime DEFAULT NULL COMMENT '撤回时间',
+  `tenant_no` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT 'zkxw' COMMENT '租户编号',
+  `order_num` int(11) DEFAULT NULL COMMENT '排序号',
+  `remark` varchar(512) COLLATE utf8mb4_unicode_ci DEFAULT NULL COMMENT '备注',
+  `create_by` varchar(64) COLLATE utf8mb4_unicode_ci DEFAULT NULL COMMENT '创建人id（评论作者）',
+  `create_dept_id` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '创建人部门ID',
+  `create_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
+  `update_by` varchar(64) COLLATE utf8mb4_unicode_ci DEFAULT NULL COMMENT '更新人id',
+  `update_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
+  `del_flag` smallint(6) NOT NULL DEFAULT '0' COMMENT '删除标记',
+  PRIMARY KEY (`id`),
+  KEY `idx_oa_comment_tenant_inst_time` (`tenant_no`,`instance_id`,`create_time`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='OA审批实例评论';