Add ProductKit binding to ConfigurableKitProductAttribute values

Implementation of kit binding feature for ConfigurableKitProduct variants:

- Added ForeignKey field `kit` to ConfigurableKitProductAttribute
  * References ProductKit with CASCADE delete
  * Optional field (blank=True, null=True)
  * Indexed for efficient queries

- Created migration 0007_add_kit_to_attribute
  * Handles existing data (NULL values for all current records)
  * Properly indexed for performance

- Updated template configurablekit_form.html
  * Injected available ProductKits into JavaScript
  * Added kit selector dropdown in card interface
  * Each value now has associated kit selection
  * JavaScript validates kit selection alongside values

- Updated JavaScript in card interface
  * serializeAttributeValues() now collects kit IDs
  * Creates parallel JSON arrays: values and kits
  * Stores in hidden fields: attributes-X-values and attributes-X-kits

- Updated views _save_attributes_from_cards() in both Create and Update
  * Reads kit IDs from POST JSON
  * Looks up ProductKit objects
  * Creates ConfigurableKitProductAttribute with FK populated
  * Gracefully handles missing kits

- Fixed _should_delete_form() method
  * More robust handling of formset deletion_field
  * Works with all formset types

- Updated __str__() method
  * Handles NULL kit case

Example workflow:
  Dlina: 50 -> Kit A, 60 -> Kit B, 70 -> Kit C
  Upakovka: BEZ -> Kit A, V_UPAKOVKE -> (no kit)

Tested with test_kit_binding.py - all tests passing
- Kit creation and retrieval
- Attribute creation with kit FK
- Mixed kit-bound and unbound attributes
- Querying attributes by kit
- Reverse queries (get kit for attribute value)

Added documentation: KIT_BINDING_IMPLEMENTATION.md

🤖 Generated with Claude Code

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

myproject/products/models/kits.py

@@ -405,9 +405,13 @@ class ConfigurableKitProduct(BaseProductEntity):
 
 class ConfigurableKitProductAttribute(models.Model):
     """
-    Атрибут родительского вариативного товара.
-    Определяет схему атрибутов для экспорта на WooCommerce и подобные площадки.
-    Например: name="Цвет", option="Красный" или name="Размер", option="M".
+    Атрибут родительского вариативного товара с привязкой к ProductKit.
+
+    Каждое значение атрибута связано с конкретным ProductKit.
+    Например:
+    - Длина: 50 → ProductKit (A)
+    - Длина: 60 → ProductKit (B)
+    - Длина: 70 → ProductKit (C)
     """
     parent = models.ForeignKey(
         ConfigurableKitProduct,
@@ -425,6 +429,15 @@ class ConfigurableKitProductAttribute(models.Model):
         verbose_name="Значение опции",
         help_text="Например: Красный, M, 60см"
     )
+    kit = models.ForeignKey(
+        ProductKit,
+        on_delete=models.CASCADE,
+        related_name='as_attribute_value_in',
+        verbose_name="Комплект для этого значения",
+        help_text="Какой ProductKit связан с этим значением атрибута",
+        blank=True,
+        null=True
+    )
     position = models.PositiveIntegerField(
         default=0,
         verbose_name="Порядок отображения",
@@ -440,14 +453,16 @@ class ConfigurableKitProductAttribute(models.Model):
         verbose_name = "Атрибут вариативного товара"
         verbose_name_plural = "Атрибуты вариативных товаров"
         ordering = ['parent', 'position', 'name', 'option']
-        unique_together = [['parent', 'name', 'option']]
+        unique_together = [['parent', 'name', 'option', 'kit']]
         indexes = [
             models.Index(fields=['parent', 'name']),
             models.Index(fields=['parent', 'position']),
+            models.Index(fields=['kit']),
         ]
 
     def __str__(self):
-        return f"{self.parent.name} - {self.name}: {self.option}"
+        kit_str = self.kit.name if self.kit else "no kit"
+        return f"{self.parent.name} - {self.name}: {self.option} ({kit_str})"
 
 
 class ConfigurableKitOption(models.Model):