feat: manage homepage banner carousel

server/migrations/011_home_banners.sql

@@ -0,0 +1,21 @@
+-- Homepage banners are independent editorial content. A banner can either
+-- reference a course (and always use its current cover) or use an uploaded
+-- image stored in Bunny Storage.
+create table home_banners (
+  id uuid primary key default gen_random_uuid(),
+  kind text not null check (kind in ('course', 'custom')),
+  course_id uuid references courses(id) on delete cascade,
+  image_url text,
+  title text not null default '',
+  description text not null default '',
+  position integer not null check (position > 0),
+  created_at timestamptz not null default now(),
+  updated_at timestamptz not null default now(),
+  constraint home_banners_source_check check (
+    (kind = 'course' and course_id is not null and image_url is null) or
+    (kind = 'custom' and course_id is null and image_url is not null)
+  ),
+  unique (position)
+);
+
+create trigger home_banners_set_updated_at before update on home_banners for each row execute function set_updated_at();