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/orders.py

@@ -270,12 +270,25 @@ def add_items(
             (o.price_delta or o.extra_cost or 0.0)
             for o in (item_in.selected_options or [])
         )
+        # Phase 2A: cost snapshot — breakdown takes priority over simple cost
+        unit_cost = None
+        if product.cost_breakdown:
+            try:
+                entries = json.loads(product.cost_breakdown)
+                total = sum(e.get("amount", 0.0) for e in entries if isinstance(e, dict))
+                if total > 0:
+                    unit_cost = total
+            except Exception:
+                pass
+        if unit_cost is None and product.cost_simple:
+            unit_cost = product.cost_simple
         item = OrderItem(
             order_id=order_id,
             product_id=item_in.product_id,
             added_by=user.id,
             quantity=item_in.quantity,
             unit_price=product.base_price + extra_cost,
+            unit_cost=unit_cost,
             selected_options=json.dumps([o.model_dump() for o in item_in.selected_options]) if item_in.selected_options else None,
             removed_ingredients=json.dumps(item_in.removed_ingredients) if item_in.removed_ingredients else None,
             notes=item_in.notes,