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/.env.example

@@ -80,6 +80,3 @@ GITLAB_PAT=
 # Generate with: openssl rand -hex 20
 GITLAB_WEBHOOK_SECRET=changeme_generate_a_random_secret
 
-# Webex Bot — used to DM users when their feedback issues are resolved.
-# Create a bot at https://developer.webex.com/my-apps and use its access token.
-WEBEX_BOT_TOKEN=