feat: implement server-side pricing calculation and add site settings management

Co-authored-by: Copilot <copilot@github.com>

supabase/migrations/09-site-settings.sql

@@ -0,0 +1,32 @@
+-- 09-site-settings.sql
+-- Site-wide settings (key-value). Used for configurable assets like hero image.
+
+create table if not exists public.site_settings (
+    key         text primary key,
+    value       text not null default '',
+    updated_at  timestamptz not null default now()
+);
+
+alter table public.site_settings enable row level security;
+
+-- Anyone can read settings (public site needs hero image URL)
+drop policy if exists "settings_public_read" on public.site_settings;
+create policy "settings_public_read"
+    on public.site_settings for select
+    using (true);
+
+-- Only authenticated admins can modify
+drop policy if exists "settings_admin_all" on public.site_settings;
+create policy "settings_admin_all"
+    on public.site_settings for all to authenticated
+    using (true) with check (true);
+
+grant select on public.site_settings to anon, authenticated, service_role;
+grant all on public.site_settings to authenticated, service_role;
+
+-- Seed default hero image
+insert into public.site_settings (key, value)
+values ('hero_image_url', '/images/ferrari-main-car.png')
+on conflict (key) do nothing;
+
+notify pgrst, 'reload schema';