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/models/order.py

@@ -81,6 +81,11 @@ class OrderItem(Base):
     # Phase 2A — cost snapshot (copied from product at time of order, never updated)
     unit_cost = Column(Float, nullable=True)
 
+    # Phase 2B — cancellation tracking
+    cancelled_by = Column(Integer, ForeignKey("users.id"), nullable=True)
+    cancelled_at = Column(DateTime(timezone=True), nullable=True)
+    cancel_reason = Column(Text, nullable=True)
+
     order = relationship("Order", back_populates="items")
     product = relationship("Product", back_populates="order_items")
     added_by_user = relationship("User", foreign_keys=[added_by], back_populates="order_items")