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/rides/partials/add_ride_modal.html

@@ -1,47 +0,0 @@
-<!-- Add Ride Modal -->
-<div id="add-ride-modal" class="fixed inset-0 z-50 hidden overflow-y-auto" aria-labelledby="modal-title" role="dialog" aria-modal="true">
-    <div class="flex items-center justify-center min-h-screen p-4">
-        <!-- Background overlay -->
-        <div class="fixed inset-0 transition-opacity bg-gray-500 bg-opacity-75" aria-hidden="true"></div>
-
-        <!-- Modal panel -->
-        <div class="relative w-full max-w-3xl p-6 mx-auto bg-white rounded-lg shadow-xl dark:bg-gray-800">
-            <div class="mb-6">
-                <h2 class="text-2xl font-bold text-gray-900 dark:text-white">
-                    Add Ride at {{ park.name }}
-                </h2>
-            </div>
-
-            <div id="modal-content">
-                {% include "rides/partials/ride_form.html" with modal=True %}
-            </div>
-        </div>
-    </div>
-</div>
-
-<!-- Modal Toggle Button -->
-<button type="button" 
-        onclick="openModal('add-ride-modal')"
-        class="inline-flex items-center px-4 py-2 text-sm font-medium text-white bg-blue-600 rounded-lg hover:bg-blue-700 dark:bg-blue-500 dark:hover:bg-blue-600">
-    <svg class="w-4 h-4 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"></path>
-    </svg>
-    Add Ride
-</button>
-
-<script>
-function openModal(modalId) {
-    document.getElementById(modalId).classList.remove('hidden');
-}
-
-function closeModal() {
-    document.getElementById('add-ride-modal').classList.add('hidden');
-}
-
-// Close modal when clicking outside
-document.getElementById('add-ride-modal').addEventListener('click', function(event) {
-    if (event.target === this) {
-        closeModal();
-    }
-});
-</script>