feat: 接入企业微信通知（临期催办 + 群机器人汇总）

新增 lib/wecom-client.ts 与 lib/wecom-notifier-service.ts，支持
群机器人 webhook 与应用消息双通道；scheduler 加入临期 N 天催办
与每日管理员汇总；action 路由补 bind_wecom_external_id 与
send_test_wecom 两个管理端动作，配套测试。

tests/wecom-notifier.test.mjs

@@ -0,0 +1,15 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { computeDueCutoff } from "../lib/wecom-notifier-service.ts";
+
+test("computeDueCutoff advances by dueDays and crosses month/year", () => {
+  assert.equal(computeDueCutoff("2026-08-18", 3), "2026-08-21");
+  assert.equal(computeDueCutoff("2026-08-30", 3), "2026-09-02");
+  assert.equal(computeDueCutoff("2026-12-30", 3), "2027-01-02");
+  assert.equal(computeDueCutoff("2026-08-18", 0), "2026-08-18");
+  assert.equal(computeDueCutoff("2026-08-18", 10), "2026-08-28");
+});
+
+test("computeDueCutoff returns input unchanged when malformed", () => {
+  assert.equal(computeDueCutoff("not-a-date", 3), "not-a-date");
+});