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

manage.py

@@ -1,27 +0,0 @@
-#!/usr/bin/env python
-"""Django's command-line utility for administrative tasks."""
-import os
-import sys
-
-
-def main():
-    """Run administrative tasks."""
-    if "test" in sys.argv and "accounts" in sys.argv:
-        os.environ.setdefault("DJANGO_SETTINGS_MODULE", "config.django.test_accounts")
-    elif "test" in sys.argv:
-        os.environ.setdefault("DJANGO_SETTINGS_MODULE", "config.django.test")
-    else:
-        os.environ.setdefault("DJANGO_SETTINGS_MODULE", "config.django.local")
-    try:
-        from django.core.management import execute_from_command_line
-    except ImportError as exc:
-        raise ImportError(
-            "Couldn't import Django. Are you sure it's installed and "
-            "available on your PYTHONPATH environment variable? Did you "
-            "forget to activate a virtual environment?"
-        ) from exc
-    execute_from_command_line(sys.argv)
-
-
-if __name__ == "__main__":
-    main()