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/accounts/turnstile_widget.html

@@ -1,22 +0,0 @@
-<script
-  src="https://challenges.cloudflare.com/turnstile/v0/api.js"
-  async
-  defer
-></script>
-<div class="turnstile">
-  <div
-    id="turnstile-widget"
-    class="cf-turnstile"
-    data-sitekey="{{ site_key }}"
-  ></div>
-</div>
-
-<script>
-  // Apply theme to the Turnstile widget based on the retrieved theme
-  document.addEventListener("DOMContentLoaded", function () {
-    const turnstileWidget = document.getElementById("turnstile-widget");
-    if (turnstileWidget) {
-      turnstileWidget.setAttribute("data-theme", theme);
-    }
-  });
-</script>