Fix self-host migrations and first admin setup

supabase/schema.sql

@@ -175,17 +175,44 @@ drop policy if exists "Users can update their own profile." on public.profiles;
 create policy "Users can update their own profile." on public.profiles
   for update using (auth.uid() = id);
 
+-- First admin setup guard for self-hosted installations
+create or replace function public.is_first_admin_setup_available()
+returns boolean
+language sql
+security definer
+set search_path = public
+as $$
+  select not exists (
+    select 1
+    from public.profiles
+    limit 1
+  );
+$$;
+
+revoke all on function public.is_first_admin_setup_available() from public;
+grant execute on function public.is_first_admin_setup_available() to anon;
+grant execute on function public.is_first_admin_setup_available() to authenticated;
+
 -- Function to handle new user signup
 create or replace function public.handle_new_user()
-returns trigger as $$
+returns trigger
+language plpgsql
+security definer
+set search_path = public
+as $$
 begin
+  if exists (select 1 from public.profiles limit 1)
+    and coalesce(new.raw_app_meta_data->>'internal_created', 'false') <> 'true' then
+    raise exception 'Registration is closed. The first admin account already exists.';
+  end if;
+
   insert into public.profiles (id, first_name, last_name, avatar_url)
   values (new.id, '', '', '')
   on conflict (id) do nothing;
 
   return new;
 end;
-$$ language plpgsql security definer;
+$$;
 
 -- Trigger to automatically create profile on signup
 drop trigger if exists on_auth_user_created on auth.users;