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/routers/products.py

@@ -235,9 +235,11 @@ def reorder_products(items: List[ProductReorderItem], db: Session = Depends(get_
 
 @router.post("/", response_model=ProductOut, status_code=status.HTTP_201_CREATED)
 def create_product(body: ProductCreate, db: Session = Depends(get_db), user: User = Depends(require_manager)):
-    data = body.model_dump(exclude={"quick_options", "options", "ingredients", "preference_sets"})
+    data = body.model_dump(exclude={"quick_options", "options", "ingredients", "preference_sets", "cost_breakdown"})
     if data.get("sort_order") == 0:
         data["sort_order"] = db.query(Product).count()
+    if body.cost_breakdown is not None:
+        data["cost_breakdown"] = json.dumps([item.model_dump() for item in body.cost_breakdown])
     product = Product(**data)
     db.add(product)
     db.flush()
@@ -277,8 +279,18 @@ def update_product(product_id: int, body: ProductUpdate, db: Session = Depends(g
     product = db.query(Product).filter(Product.id == product_id).first()
     if not product:
         raise HTTPException(status_code=404, detail="Product not found")
-    for field, value in body.model_dump(exclude_none=True, exclude={"quick_options", "options", "ingredients", "preference_sets"}).items():
+    scalar_fields = body.model_dump(
+        exclude_none=True,
+        exclude={"quick_options", "options", "ingredients", "preference_sets", "cost_breakdown"},
+    )
+    for field, value in scalar_fields.items():
         setattr(product, field, value)
+    # cost_breakdown is a list of objects — serialize to JSON for storage
+    if body.cost_breakdown is not None:
+        product.cost_breakdown = json.dumps([item.model_dump() for item in body.cost_breakdown])
+    elif "cost_breakdown" in body.model_fields_set:
+        # explicitly set to null — clear it
+        product.cost_breakdown = None
     if body.quick_options is not None:
         _replace_quick_options(db, product, body.quick_options)
     if body.options is not None: