fix: use phone_number as USERNAME_FIELD on User model

- USERNAME_FIELD = "phone_number" (was "email") — email is optional on
  this platform; most customers will be phone-only
- Add REQUIRED_FIELDS = [] to make the intent explicit
- Update create_superuser to accept phone_number as the identifier and
  pass it through to create_user as a keyword argument
- All 35 backend tests pass

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

backend/apps/accounts/models.py

@@ -29,7 +29,7 @@ class UserManager(BaseUserManager):
         user.save(using=self._db)
         return user
 
-    def create_superuser(self, email, password=None, **extra_fields):
+    def create_superuser(self, phone_number, password=None, **extra_fields):
         extra_fields.setdefault("is_staff", True)
         extra_fields.setdefault("is_superuser", True)
         extra_fields.setdefault("role", UserRole.ADMIN)
@@ -37,7 +37,7 @@ class UserManager(BaseUserManager):
             raise ValueError("Superuser must have is_staff=True")
         if extra_fields.get("is_superuser") is not True:
             raise ValueError("Superuser must have is_superuser=True")
-        return self.create_user(email, password, **extra_fields)
+        return self.create_user(phone_number=phone_number, password=password, **extra_fields)
 
 
 class User(AbstractBaseUser, PermissionsMixin):
@@ -59,7 +59,8 @@ class User(AbstractBaseUser, PermissionsMixin):
 
     objects = UserManager()
 
-    USERNAME_FIELD = "email"
+    USERNAME_FIELD = "phone_number"
+    REQUIRED_FIELDS = []  # email is optional; phone_number is the identifier
 
     def __str__(self):
         return self.email or self.phone_number or str(self.id)