feat: major project restructure - move Django to backend dir and fix critical imports

- Restructure project: moved Django backend to backend/ directory
- Add frontend/ directory for future Next.js application
- Add shared/ directory for common resources
- Fix critical Django import errors:
  - Add missing sys.path modification for apps directory
  - Fix undefined CATEGORY_CHOICES imports in rides module
  - Fix media migration undefined references
  - Remove unused imports and f-strings without placeholders
- Install missing django-environ dependency
- Django server now runs without ModuleNotFoundError
- Update .gitignore and README for new structure
- Add pnpm workspace configuration for monorepo setup

templates/parks/partials/park_search_results.html

@@ -1,27 +0,0 @@
-<div class="absolute z-50 w-full mt-1 bg-white border border-gray-300 rounded-lg shadow-lg dark:bg-gray-700 dark:border-gray-600" style="max-height: 240px; overflow-y: auto;">
-    {% if parks %}
-        {% for park in parks %}
-            <button type="button"
-                    class="w-full px-4 py-2 text-left text-gray-900 hover:bg-gray-100 dark:text-white dark:hover:bg-gray-600"
-                    onclick="selectPark('{{ park.id }}', '{{ park.name|escapejs }}')">
-                {{ park.name }}
-            </button>
-        {% endfor %}
-    {% else %}
-        <div class="px-4 py-2 text-gray-700 dark:text-gray-300">
-            {% if search_term %}
-                No parks found
-            {% else %}
-                Start typing to search...
-            {% endif %}
-        </div>
-    {% endif %}
-</div>
-
-<script>
-function selectPark(id, name) {
-    document.getElementById('id_park').value = id;
-    document.getElementById('id_park_search').value = name;
-    document.getElementById('park-search-results').innerHTML = '';
-}
-</script>