v0.3.5: restore full push notification system (14 channels)

Restored from v0.2.4:
- notifiers/: 14 push channels (bark/serverchan/telegram/lark/webhook/wechat/discord/smtp/...)
- push-user.service.ts: multi-user push config linked to cloud_configs.promotion_account
- notification.service.ts: full dispatcher with per-config + global fallback

New integrations:
- cloud.service.ts: notifyConfigEvent on save_success/cookie_expire/save_fail
- admin.routes.ts: 7 new API endpoints for push users, notify providers, channel test
- database.ts: migration for cloud_configs.notify_config column

How it works:
- Configure push channels in /admin/system-configs (global_notify_config JSON)
- Or per-cloud: link push_users.account = cloud_configs.promotion_account
- Notify events: save_success (green), cookie_expire (red), save_fail >=3 consecutive (yellow)

source_clean/src/cloud/notifiers/webhook.notifier.ts

@@ -0,0 +1,33 @@
+import { Notifier, NotifyParams, NotifyResult, NotifierParam } from './notifier.types';
+
+const params: NotifierParam[] = [
+  { key: 'url', label: 'Webhook URL', type: 'url', required: true, placeholder: 'https://example.com/webhook' },
+  { key: 'title', label: '\u6807\u9898', type: 'text', default: 'CloudSearch', required: false },
+  { key: 'content', label: '\u5185\u5bb9', type: 'text', required: true },
+  { key: 'level', label: '\u7ea7\u522b', type: 'text', default: 'info', required: false },
+];
+
+export const webhookNotifier: Notifier = {
+  name: 'webhook',
+  label: '\u81ea\u5b9a\u4e49 Webhook',
+  params,
+  async notify(p: NotifyParams): Promise<NotifyResult> {
+    try {
+      const resp = await fetch(p.url, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          title: p.title || 'CloudSearch',
+          content: p.content || '',
+          level: p.level || 'info',
+          source: 'CloudSearch',
+          timestamp: new Date().toISOString(),
+        }),
+      });
+      if (resp.ok) return { success: true, message: 'Webhook \u63a8\u9001\u6210\u529f' };
+      return { success: false, message: `HTTP ${resp.status}` };
+    } catch (err: any) {
+      return { success: false, message: err.message };
+    }
+  },
+};