feat: Add signal handler for synchronizing Incoming edits with StockBatch

## Changes

### 1. Fixed missing signal handler for Incoming edit (inventory/signals.py)
- Added new signal handler `update_stock_batch_on_incoming_edit()` that:
  - Triggers when Incoming is edited (created=False)
  - Synchronizes StockBatch with new quantity and cost_price values
  - Automatically triggers cost price recalculation for the product
  - Updates Stock (inventory balance) for the warehouse
  - Includes proper logging and error handling

### 2. Created IncomingModelForm for editing individual incoming items (inventory/forms.py)
- New ModelForm: `IncomingModelForm` that:
  - Inherits from forms.ModelForm (accepts 'instance' parameter required by UpdateView)
  - Allows editing: product, quantity, cost_price, notes
  - Includes validation for positive quantity and non-negative cost_price
  - Filters only active products

### 3. Updated IncomingUpdateView (inventory/views/incoming.py)
- Changed form_class from IncomingForm to IncomingModelForm
- Updated imports to include IncomingModelForm
- Removed obsolete comments from form_valid method

## Architecture

When editing an Incoming item:
1. User submits form with new quantity/cost_price
2. form.save() triggers post_save signal (created=False)
3. update_stock_batch_on_incoming_edit() synchronizes StockBatch
4. StockBatch.save() triggers update_product_cost_on_batch_change()
5. Product.cost_price is recalculated with weighted average

## Problem Solved

Previously, editing an Incoming item would NOT:
- Update the related StockBatch
- Recalculate product cost_price
- Update warehouse inventory balance
- Maintain data consistency between Incoming and StockBatch

Now all these operations happen automatically through the signal chain.

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

myproject/inventory/forms.py

@@ -13,12 +13,31 @@ from products.models import Product
 class WarehouseForm(forms.ModelForm):
     class Meta:
         model = Warehouse
-        fields = ['name', 'description', 'is_active', 'is_default']
+        fields = [
+            'name',
+            'description',
+            'street',
+            'building_number',
+            'phone',
+            'email',
+            'is_active',
+            'is_default',
+            'is_pickup_point'
+        ]
         widgets = {
-            'name': forms.TextInput(attrs={'class': 'form-control'}),
-            'description': forms.Textarea(attrs={'class': 'form-control', 'rows': 4}),
+            'name': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'Название склада'}),
+            'description': forms.Textarea(attrs={'class': 'form-control', 'rows': 3, 'placeholder': 'Описание'}),
+            'street': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'Улица'}),
+            'building_number': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'Номер дома'}),
+            'phone': forms.TextInput(attrs={'class': 'form-control', 'placeholder': '+375 (29) 123-45-67'}),
+            'email': forms.EmailInput(attrs={'class': 'form-control', 'placeholder': 'email@example.com'}),
             'is_active': forms.CheckboxInput(attrs={'class': 'form-check-input'}),
             'is_default': forms.CheckboxInput(attrs={'class': 'form-check-input'}),
+            'is_pickup_point': forms.CheckboxInput(attrs={'class': 'form-check-input'}),
+        }
+        help_texts = {
+            'is_default': 'Автоматически выбирается при создании новых документов',
+            'is_pickup_point': 'Можно ли выбрать этот склад как точку самовывоза заказа',
         }
 
 
@@ -308,6 +327,41 @@ class IncomingForm(forms.Form):
         return document_number
 
 
+class IncomingModelForm(forms.ModelForm):
+    """
+    ModelForm для редактирования отдельного товара в поступлении (Incoming).
+    Используется в IncomingUpdateView для редактирования существующих товаров.
+    """
+    class Meta:
+        model = Incoming
+        fields = ['product', 'quantity', 'cost_price', 'notes']
+        widgets = {
+            'product': forms.Select(attrs={'class': 'form-control'}),
+            'quantity': forms.NumberInput(attrs={'class': 'form-control', 'step': '0.001'}),
+            'cost_price': forms.NumberInput(attrs={'class': 'form-control', 'step': '0.01'}),
+            'notes': forms.Textarea(attrs={'class': 'form-control', 'rows': 3}),
+        }
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        # Фильтруем только активные товары
+        self.fields['product'].queryset = Product.objects.filter(
+            is_active=True
+        ).order_by('name')
+
+    def clean_quantity(self):
+        quantity = self.cleaned_data.get('quantity')
+        if quantity and quantity <= 0:
+            raise ValidationError('Количество должно быть больше нуля')
+        return quantity
+
+    def clean_cost_price(self):
+        cost_price = self.cleaned_data.get('cost_price')
+        if cost_price and cost_price < 0:
+            raise ValidationError('Цена не может быть отрицательной')
+        return cost_price
+
+
 # ============================================================================
 # TRANSFER FORMS - Перемещение товаров между складами
 # ============================================================================