fix: report cancellation bugs + missing OrderItem model columns

Backend:
- OrderItem model: add cancelled_at, cancelled_by, cancel_reason columns
  (migration existed but model was missing them — caused 500 on cancellations_log)
- shifts.py _enrich_shift: count cancellations by quantity sum (not row count),
  add cancellation_value (sum of unit_price * quantity)
- reports.py current business day: add cancelled_items count (per-item quantity sum,
  across all orders in the day, not just fully-cancelled orders)

Manager dashboard:
- PrintFontsTab: fix SSE auth token key (access_token → manager_token, was causing 403)
- Today: show cancelled_items count as sub-label on Ακυρώσεις stat card
- OrderHistory: items/cancellations columns now use quantity sum, not row count
- WorkDaySummary drill-down: same quantity-sum fix
- ShiftsOverview: add Αξία Ακυρ. column next to Ακυρώσεις

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

manager_dashboard/src/pages/Settings/tabs/PrintFontsTab.jsx

@@ -804,7 +804,7 @@ function FindPrintersModal({ onClose, onSelect }) {
     setError(null)
     setScanning(true)
 
-    const token = localStorage.getItem('access_token') || ''
+    const token = localStorage.getItem('manager_token') || ''
     const params = new URLSearchParams({ subnet, port })
     // Use fetch with ReadableStream to consume SSE (EventSource doesn't support auth headers)
     fetch(`/api/system/printers/scan?${params}`, {