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/schemas/product.py

@@ -255,6 +255,11 @@ class PreferenceSetOut(BaseModel):
 
 # ── Products ──────────────────────────────────────────────────────────────────
 
+class CostBreakdownItem(BaseModel):
+    label: str
+    amount: float
+
+
 class ProductBase(BaseModel):
     name: str
     description: Optional[str] = None
@@ -272,6 +277,9 @@ class ProductBase(BaseModel):
     digital_price: Optional[float] = None
     digital_discount: Optional[float] = None
     digital_image_url: Optional[str] = None
+    # Phase 2A — cost tracking
+    cost_simple: Optional[float] = None
+    cost_breakdown: Optional[List[CostBreakdownItem]] = None
 
 
 class ProductCreate(ProductBase):
@@ -302,6 +310,9 @@ class ProductUpdate(BaseModel):
     digital_price: Optional[float] = None
     digital_discount: Optional[float] = None
     digital_image_url: Optional[str] = None
+    # Phase 2A — cost tracking
+    cost_simple: Optional[float] = None
+    cost_breakdown: Optional[List[CostBreakdownItem]] = None
 
 
 class ProductReorderItem(BaseModel):
@@ -318,3 +329,42 @@ class ProductOut(ProductBase):
     image_url: Optional[str] = None
 
     model_config = {"from_attributes": True}
+
+    @model_validator(mode='before')
+    @classmethod
+    def parse_cost_breakdown(cls, data: Any) -> Any:
+        if hasattr(data, 'cost_breakdown'):
+            raw = data.cost_breakdown
+            parsed = None
+            if isinstance(raw, str):
+                try:
+                    parsed = json.loads(raw)
+                except Exception:
+                    parsed = None
+            # Return a plain dict so other validators still see all fields
+            return {
+                'id': data.id,
+                'name': data.name,
+                'description': data.description,
+                'category_id': data.category_id,
+                'base_price': data.base_price,
+                'is_available': data.is_available,
+                'lifecycle_status': data.lifecycle_status,
+                'printer_zone_id': data.printer_zone_id,
+                'sort_order': data.sort_order,
+                'image_url': data.image_url,
+                'digital_visible': bool(data.digital_visible),
+                'digital_available': bool(data.digital_available),
+                'digital_name': data.digital_name,
+                'digital_description': data.digital_description,
+                'digital_price': data.digital_price,
+                'digital_discount': data.digital_discount,
+                'digital_image_url': data.digital_image_url,
+                'cost_simple': data.cost_simple,
+                'cost_breakdown': parsed,
+                'quick_options': list(data.quick_options),
+                'options': list(data.options),
+                'ingredients': list(data.ingredients),
+                'preference_sets': list(data.preference_sets),
+            }
+        return data