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

templates/catalog/detail.html

@@ -37,7 +37,9 @@
         </section>
 
         <section class="product-summary" aria-label="Информация о товаре">
-            <p class="product-summary__category">{{ item.category.name }}</p>
+            <p class="product-summary__category">
+                {{ item.category.name }}{% if item.subcategory %} / {{ item.subcategory.name }}{% endif %}
+            </p>
             <h1 class="product-summary__title">{{ item.name }}</h1>
 
             <div class="product-summary__meta">