supabase: extend initial migration and add first campaigns test

supabase/migrations/20250926150848_campaigns.sql

@@ -0,0 +1,19 @@
+create table public.campaigns (
+    id uuid primary key default gen_random_uuid(),
+    created_at timestamp with time zone not null default now(),
+    brand_id uuid not null references public.brands(user_id) on delete cascade,
+    description text not null default ''::text
+);
+alter table public.campaigns enable row level security;
+create policy everyone_view_only
+    on public.campaigns
+    as permissive
+    for select
+    to authenticated, anon
+    using (true);
+create policy users_manage_their_own_data
+    on public.campaigns
+    as permissive
+    for all
+    to authenticated
+    using ((( SELECT auth.uid() AS uid) = brand_id));