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/moderation/partials/photo_submission_content.html

@@ -1,23 +0,0 @@
-{% load static %}
-
-<div class="space-y-6">
-    <div class="p-6 bg-white border rounded-lg shadow-lg dark:bg-gray-800 border-gray-200/50 dark:border-gray-700/50">
-        <div class="flex items-center justify-between mb-6">
-            <h3 class="text-lg font-medium text-gray-900 dark:text-white">Photo Submissions</h3>
-            <span class="px-3 py-1 text-sm font-medium text-yellow-800 bg-yellow-100 rounded-full dark:bg-yellow-900/50 dark:text-yellow-200">
-                {{ submissions|length }}
-            </span>
-        </div>
-
-        <div class="grid grid-cols-1 gap-6 md:grid-cols-2 lg:grid-cols-3">
-            {% for submission in submissions %}
-                {% include "moderation/partials/photo_submission.html" %}
-            {% empty %}
-                <div class="py-8 text-center col-span-full">
-                    <i class="mb-3 text-4xl text-gray-400 fas fa-camera"></i>
-                    <p class="text-gray-600 dark:text-gray-400">No photo submissions found</p>
-                </div>
-            {% endfor %}
-        </div>
-    </div>
-</div>