feat: Phase 2A — product cost tracking

- Products: add cost_simple (flat €) and cost_breakdown (JSON line items)
- OrderItems: add unit_cost snapshot written at time of order creation
- Snapshot logic: breakdown sum takes priority over cost_simple; NULL if neither set
- Product schema: serialize/deserialize cost_breakdown as JSON; expose in API
- Product performance report: add trackable_profit, uncosted_revenue, has_gap per product
- Manager UI: cost section in product edit form (simple / detailed toggle with live margin %)
- Products list: show margin % or cost-error badge per product
- Product performance report: profit column + gap warning banner

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

local_backend/models/order.py

@@ -75,6 +75,9 @@ class OrderItem(Base):
     payment_method = Column(String, nullable=True)  # 'cash'|'card'|'other' — future use
     paid_in_shift_id = Column(Integer, ForeignKey("waiter_shifts.id"), nullable=True)
 
+    # Phase 2A — cost snapshot (copied from product at time of order, never updated)
+    unit_cost = Column(Float, 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")