feat: add subcategory support for catalog and admin

- Introduced `Subcategory` model with migration and unique slug constraint per category
- Enabled subcategory assignment for items with validation to ensure category matching
- Updated catalog views to handle subcategories, filtering items accordingly
- Enhanced templates (`index.html`, `side-bar.html`, `item-list.html`, `detail.html`) to display subcategories
- Added dynamic subcategory dropdown in Django admin with category filtering
- Created JavaScript for admin subcategory handling and AJAX population
- Wrote comprehensive tests for subcategory functionality in views and admin

items/migrations/0003_subcategory_item_subcategory_and_more.py

@@ -0,0 +1,37 @@
+# Generated by Django 6.0.4 on 2026-04-26 20:49
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('items', '0002_itemimage'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Subcategory',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=255, verbose_name='Название')),
+                ('slug', models.SlugField(verbose_name='URL')),
+                ('category', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='subcategories', to='items.category', verbose_name='Категория')),
+            ],
+            options={
+                'verbose_name': 'Подкатегория',
+                'verbose_name_plural': 'Подкатегории',
+                'ordering': ('category__name', 'name'),
+            },
+        ),
+        migrations.AddField(
+            model_name='item',
+            name='subcategory',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='items', to='items.subcategory', verbose_name='Подкатегория'),
+        ),
+        migrations.AddConstraint(
+            model_name='subcategory',
+            constraint=models.UniqueConstraint(fields=('category', 'slug'), name='unique_subcategory_slug_per_category'),
+        ),
+    ]