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

static/items/admin/item-subcategory.js

@@ -0,0 +1,67 @@
+(function () {
+    function getSubcategoriesUrl() {
+        return window.location.pathname.replace(/\/(?:add|\d+\/change)\/?$/, '/subcategories/');
+    }
+
+    function setOptions(select, subcategories, selectedId) {
+        var emptyOption = Array.prototype.find.call(select.options, function (option) {
+            return option.value === '';
+        });
+        var emptyLabel = emptyOption ? emptyOption.textContent : '---------';
+
+        select.innerHTML = '';
+        select.appendChild(new Option(emptyLabel, ''));
+
+        subcategories.forEach(function (subcategory) {
+            var option = new Option(subcategory.name, subcategory.id);
+
+            if (String(subcategory.id) === String(selectedId)) {
+                option.selected = true;
+            }
+
+            select.appendChild(option);
+        });
+    }
+
+    function loadSubcategories(categorySelect, subcategorySelect, selectedId) {
+        if (!categorySelect.value) {
+            setOptions(subcategorySelect, [], '');
+            return;
+        }
+
+        var url = getSubcategoriesUrl() + '?category_id=' + encodeURIComponent(categorySelect.value);
+
+        fetch(url, {
+            headers: {
+                'X-Requested-With': 'XMLHttpRequest'
+            }
+        })
+            .then(function (response) {
+                if (!response.ok) {
+                    throw new Error('Failed to load subcategories.');
+                }
+
+                return response.json();
+            })
+            .then(function (data) {
+                setOptions(subcategorySelect, data.subcategories || [], selectedId);
+            });
+    }
+
+    document.addEventListener('DOMContentLoaded', function () {
+        var categorySelect = document.getElementById('id_category');
+        var subcategorySelect = document.getElementById('id_subcategory');
+
+        if (!categorySelect || !subcategorySelect) {
+            return;
+        }
+
+        categorySelect.addEventListener('change', function () {
+            loadSubcategories(categorySelect, subcategorySelect, '');
+        });
+
+        if (!categorySelect.value) {
+            setOptions(subcategorySelect, [], '');
+        }
+    });
+}());