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/migrations/add_notifications_table.js

@@ -0,0 +1,45 @@
+const pool = require('../db');
+
+async function run() {
+    console.log('Starting notifications table migration...');
+    try {
+        await pool.query(`
+            CREATE TABLE IF NOT EXISTS notifications (
+                id SERIAL PRIMARY KEY,
+                user_id INTEGER REFERENCES users(id),
+                username TEXT NOT NULL,
+                type TEXT NOT NULL DEFAULT 'issue_resolved',
+                title TEXT NOT NULL,
+                message TEXT NOT NULL,
+                issue_number INTEGER,
+                read BOOLEAN DEFAULT FALSE,
+                created_at TIMESTAMP DEFAULT NOW()
+            )
+        `);
+        console.log('✓ notifications table created (or already exists)');
+
+        await pool.query(`
+            CREATE INDEX IF NOT EXISTS idx_notifications_username
+            ON notifications(username)
+        `);
+        console.log('✓ username index created');
+
+        await pool.query(`
+            CREATE INDEX IF NOT EXISTS idx_notifications_read
+            ON notifications(username, read)
+        `);
+        console.log('✓ username/read index created');
+
+        console.log('Migration complete.');
+    } catch (err) {
+        console.error('Migration failed:', err.message);
+        throw err;
+    }
+}
+
+module.exports = { run };
+
+// Self-execute when run directly
+if (require.main === module) {
+    run().then(() => process.exit(0)).catch(() => process.exit(1));
+}