refactor: reorganizar estructura del proyecto al estándar app/api_config

- core/ → app/api_config/
- apps/backend_admin/ → app/backend_admin/
- apps/common/ → app/common/
- apps/promociones/ → app/promociones/
- manage.py → app/manage.py
- Añadir app/requirements.txt
- Actualizar todos los imports y referencias (DJANGO_SETTINGS_MODULE, ROOT_URLCONF, WSGI_APPLICATION, INSTALLED_APPS)
- Actualizar Dockerfile con nuevo WORKDIR

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/manage.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+"""Django's command-line utility for administrative tasks."""
+import os
+import sys
+
+def main():
+    """Run administrative tasks."""
+    # Apuntamos a la configuración dentro de la carpeta 'core'
+    os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'api_config.settings')
+    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()