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/product.py

@@ -46,6 +46,10 @@ class Product(Base):
     digital_discount = Column(Float, default=0.0, nullable=False)
     digital_image_url = Column(String, nullable=True)
 
+    # Phase 2A — cost tracking
+    cost_simple = Column(Float, nullable=True)
+    cost_breakdown = Column(Text, nullable=True)  # JSON: [{"label": str, "amount": float}]
+
     category = relationship("Category", back_populates="products")
     printer_zone = relationship("Printer", back_populates="products")
     quick_options = relationship("ProductQuickOption", back_populates="product", cascade="all, delete-orphan")