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/migrations/0007_add_kit_to_attribute.py

@@ -0,0 +1,31 @@
+# Generated by Django 5.0.10 on 2025-11-18 18:13
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('products', '0006_add_configurablekitoptionattribute'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='configurablekitproductattribute',
+            unique_together=set(),
+        ),
+        migrations.AddField(
+            model_name='configurablekitproductattribute',
+            name='kit',
+            field=models.ForeignKey(blank=True, help_text='Какой ProductKit связан с этим значением атрибута', null=True, on_delete=django.db.models.deletion.CASCADE, related_name='as_attribute_value_in', to='products.productkit', verbose_name='Комплект для этого значения'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='configurablekitproductattribute',
+            unique_together={('parent', 'name', 'option', 'kit')},
+        ),
+        migrations.AddIndex(
+            model_name='configurablekitproductattribute',
+            index=models.Index(fields=['kit'], name='products_co_kit_id_c5d506_idx'),
+        ),
+    ]