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/404.html

@@ -1,15 +0,0 @@
-{% extends "base/base.html" %}
-
-{% block title %}Page Not Found{% endblock %}
-
-{% block content %}
-<div class="container px-4 py-16 mx-auto">
-    <div class="text-center">
-        <h1 class="mb-4 text-4xl font-bold">404 - Page Not Found</h1>
-        <p class="mb-8 text-lg">The page you're looking for doesn't exist or has been moved. Please try again in a while.</p>
-        <a href="/" class="inline-block px-4 py-2 font-bold text-white bg-blue-600 rounded hover:bg-blue-700">
-            Return Home
-        </a>
-    </div>
-</div>
-{% endblock %}