Make first user of instance automatically super_admin (#655)

* Implement dynamic role assignment for new family creators.

Introduced `User.role_for_new_family_creator` to assign `super_admin` to the first user of an instance and a configurable fallback role (e.g., `admin`) to subsequent users. Updated controllers and tests accordingly.

* Update default fallback role for family creators to admin.

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Co-authored-by: luckyPipewrench <luckypipewrench@proton.me>

app/controllers/api/v1/auth_controller.rb

@@ -36,9 +36,10 @@ module Api
         user = User.new(user_signup_params)
 
         # Create family for new user
+        # First user of an instance becomes super_admin
         family = Family.new
         user.family = family
-        user.role = :admin
+        user.role = User.role_for_new_family_creator
 
         if user.save
           # Claim invite code if provided