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/_fragments/item-list.html

@@ -14,7 +14,9 @@
                 {% endwith %}
 
                 <div class="item-card-body">
-                    <div class="item-card-category">{{ item.category.name }}</div>
+                    <div class="item-card-category">
+                        {{ item.category.name }}{% if item.subcategory %} / {{ item.subcategory.name }}{% endif %}
+                    </div>
                     <h2 class="item-card-title">{{ item.name }}</h2>
                     <div class="item-card-footer">
                         <span class="item-card-price">{{ item.price }} руб.</span>