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/reports.py

@@ -950,16 +950,29 @@ def product_performance(
                 "category_id": product.category_id if product else None,
                 "qty_sold": 0,
                 "revenue": 0.0,
+                "trackable_profit": 0.0,
+                "uncosted_revenue": 0.0,
+                "uncosted_items": 0,
                 "order_ids": set(),
             }
-        summary[pid]["qty_sold"] += item.quantity
-        summary[pid]["revenue"] += item.unit_price * item.quantity
+        qty = item.quantity
+        revenue = item.unit_price * qty
+        summary[pid]["qty_sold"] += qty
+        summary[pid]["revenue"] += revenue
         summary[pid]["order_ids"].add(item.order_id)
+        if item.unit_cost is not None:
+            summary[pid]["trackable_profit"] += revenue - (item.unit_cost * qty)
+        else:
+            summary[pid]["uncosted_revenue"] += revenue
+            summary[pid]["uncosted_items"] += qty
 
     result = []
     for entry in summary.values():
         entry["order_count"] = len(entry.pop("order_ids"))
         entry["revenue"] = round(entry["revenue"], 2)
+        entry["trackable_profit"] = round(entry["trackable_profit"], 2)
+        entry["uncosted_revenue"] = round(entry["uncosted_revenue"], 2)
+        entry["has_gap"] = entry["uncosted_items"] > 0
         result.append(entry)
 
     result.sort(key=lambda x: x["qty_sold"], reverse=True)