Replace Webex bot with in-app notification system

Org blocks external Webex bots, so replaced the DM approach with an in-app
notification bell. GitLab webhook still fires on issue close, but now writes
to a notifications table instead of calling Webex API.

- New: notifications table + migration
- New: GET/PATCH/POST /api/notifications endpoints
- New: NotificationBell component (bell icon + badge + dropdown)
- Removed: backend/helpers/webexBot.js (org-blocked)
- Removed: WEBEX_BOT_TOKEN from .env

backend/server.js

@@ -34,6 +34,7 @@ const createJiraTicketsRouter         = require('./routes/jiraTickets');
 const createCardApiRouter             = require('./routes/cardApi');
 const createFeedbackRouter            = require('./routes/feedback');
 const createWebhooksRouter            = require('./routes/webhooks');
+const createNotificationsRouter       = require('./routes/notifications');
 
 const app = express();
 const PORT = process.env.PORT || 3001;
@@ -231,6 +232,9 @@ app.use('/api/card', createCardApiRouter());
 // Feedback routes — bug reports and feature requests to GitLab
 app.use('/api/feedback', createFeedbackRouter());
 
+// In-app notifications routes (authenticated users)
+app.use('/api/notifications', createNotificationsRouter());
+
 // GitLab webhook routes — receives issue lifecycle events (no auth required)
 app.use('/api/webhooks', createWebhooksRouter());