feat: add custom authentication backend to support login with email or username

- Implemented `EmailOrUsernameBackend` to allow authentication via email or username
- Updated settings to include the new authentication backend
- Modified tests to cover email-based login scenarios
- Fixed user lookup logic in forms to handle case-insensitive email matching

santehray/settings.py

@@ -192,6 +192,10 @@ INSTALLED_APPS = [
     'users',
 ]
 
+AUTHENTICATION_BACKENDS = [
+    "users.auth_backends.EmailOrUsernameBackend",
+]
+
 MIDDLEWARE = [
     'django.middleware.security.SecurityMiddleware',
     'whitenoise.middleware.WhiteNoiseMiddleware',