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>

local_backend/routers/shifts.py

@@ -75,7 +75,7 @@ def _enrich_shift(shift: WaiterShift, db: Session) -> dict:
     wname = (w.full_name or w.username) if w else f"#{shift.waiter_id}"
     total = compute_shift_total(shift.id, db) if shift.ended_at is None else (shift.total_collected or 0.0)
     pay_data = compute_shift_pay(shift)
-    # Count cancelled items attributed to this waiter during shift window
+    # Count cancelled items and their value attributed to this waiter during shift window
     cancelled_q = db.query(OrderItem).filter(
         OrderItem.status == "cancelled",
         OrderItem.added_by == shift.waiter_id,
@@ -83,7 +83,9 @@ def _enrich_shift(shift: WaiterShift, db: Session) -> dict:
     )
     if shift.ended_at:
         cancelled_q = cancelled_q.filter(OrderItem.added_at <= shift.ended_at)
-    cancellations = cancelled_q.count()
+    cancelled_rows = cancelled_q.all()
+    cancellations = sum(r.quantity for r in cancelled_rows)
+    cancellation_value = round(sum(r.unit_price * r.quantity for r in cancelled_rows), 2)
     return {
         "id": shift.id,
         "waiter_id": shift.waiter_id,
@@ -100,6 +102,7 @@ def _enrich_shift(shift: WaiterShift, db: Session) -> dict:
         "duration_hours": pay_data["duration_hours"],
         "shift_pay": pay_data["shift_pay"],
         "cancellations": cancellations,
+        "cancellation_value": cancellation_value,
         # Phase 2E
         "counted_cash_end": shift.counted_cash_end,
         "cash_discrepancy": shift.cash_discrepancy,