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/views/incoming.py

@@ -10,7 +10,7 @@ from django.views.decorators.http import require_http_methods
 from django.utils.decorators import method_decorator
 from django.db import IntegrityError, transaction
 from ..models import Incoming, IncomingBatch, Warehouse
-from ..forms import IncomingForm, IncomingLineForm
+from ..forms import IncomingForm, IncomingLineForm, IncomingModelForm
 from inventory.utils import generate_incoming_document_number
 from products.models import Product
 
@@ -47,13 +47,11 @@ class IncomingUpdateView(LoginRequiredMixin, UpdateView):
     Обработанные приходы редактировать нельзя.
     """
     model = Incoming
-    form_class = IncomingForm
+    form_class = IncomingModelForm
     template_name = 'inventory/incoming/incoming_form.html'
     success_url = reverse_lazy('inventory:incoming-list')
 
     def form_valid(self, form):
-        # При редактировании можем оставить номер пустым - модель генерирует при сохранении
-        # Но это только если объект ещё не имеет номера (новый)
         messages.success(self.request, f'Приход товара обновлён.')
         return super().form_valid(form)