Add screenshot uploads to feedback modal, Webex bot DM on issue close

- Feedback modal now supports up to 3 image attachments (PNG/JPG/GIF/WebP, 5MB
  each) with thumbnail previews. Images are uploaded to GitLab project uploads
  and embedded as markdown in the issue description.
- New webhook endpoint (POST /api/webhooks/gitlab) receives issue close events,
  parses the submitter from the description, looks up their email, and sends a
  Webex DM via the Patches O'Houlihan bot.
- New helper: backend/helpers/webexBot.js (fire-and-forget DM sender).
- Requires WEBEX_BOT_TOKEN and GITLAB_WEBHOOK_SECRET in backend/.env.

backend/.env.example

@@ -74,3 +74,12 @@ DATABASE_URL=postgresql://steam:<password>@localhost:5433/cve_dashboard
 GITLAB_URL=http://steam-gitlab.charterlab.com
 GITLAB_PROJECT_ID=
 GITLAB_PAT=
+
+# GitLab Webhook Secret — shared secret for validating incoming webhook requests.
+# Set this same value in GitLab project > Settings > Webhooks > Secret Token.
+# 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=