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

frontend/src/App.js

@@ -19,6 +19,7 @@ import JiraPage from './components/pages/JiraPage';
 import AdminPage from './components/pages/AdminPage';
 import ArchiveSummaryBar from './components/pages/ArchiveSummaryBar';
 import FeedbackModal from './components/FeedbackModal';
+import NotificationBell from './components/NotificationBell';
 import './App.css';
 
 const API_BASE = process.env.REACT_APP_API_BASE || 'http://localhost:3001/api';
@@ -1051,6 +1052,7 @@ export default function App() {
                 <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><path d="m8 2 1.88 1.88"/><path d="M14.12 3.88 16 2"/><path d="M9 7.13v-1a3.003 3.003 0 1 1 6 0v1"/><path d="M12 20c-3.3 0-6-2.7-6-6v-3a4 4 0 0 1 4-4h4a4 4 0 0 1 4 4v3c0 3.3-2.7 6-6 6"/><path d="M12 20v-9"/><path d="M6.53 9C4.6 8.8 3 7.1 3 5"/><path d="M6 13H2"/><path d="M3 21c0-2.1 1.7-3.9 3.8-4"/><path d="M20.97 5c0 2.1-1.6 3.8-3.5 4"/><path d="M22 13h-4"/><path d="M17.2 17c2.1.1 3.8 1.9 3.8 4"/></svg>
                 Bug
               </button>
+              <NotificationBell />
               <UserMenu onManageUsers={() => setShowUserManagement(true)} onAuditLog={() => setShowAuditLog(true)} onFeatureRequest={() => { setFeedbackType('feature'); setShowFeedback(true); }} />
             </div>
           </div>