Mobile scanner redesign + backend live search

- Scanner page: fullscreen mobile-first layout, Scan/Search/Recent tabs
- Scan tab: auto-start camera, switch camera, vibration/sound feedback
- Valid/invalid fullscreen states, confirm check-in, auto-return to camera
- Search tab: live backend search (300ms debounce), tap card for detail + check-in
- Recent tab: last 20 check-ins, session counter
- Backend: GET /api/tickets/search (live search), GET /api/tickets/stats/checkin
- Admin layout: hide sidebar on scanner page; fix hooks order (no early return before useEffect)
- Back button to dashboard/events (staff → events, others → admin)
- API: searchLive, getCheckinStats, LiveSearchResult; PostgreSQL LOWER cast for UUID

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/.env.example

@@ -72,3 +72,4 @@ SMTP_TLS_REJECT_UNAUTHORIZED=true
 # Maximum number of emails that can be sent per hour (default: 30)
 # If the limit is reached, queued emails will pause and resume automatically
 MAX_EMAILS_PER_HOUR=30
+