feat(db): Add Postgres function to atomically update default image (#339)

Author: iambriccardo

etl-api/migrations/20250905100000_update_default_image_function.sql

@@ -0,0 +1,36 @@
+-- Function to atomically create (if needed) and set the default image by name.
+-- Usage: select app.update_default_image('supabase/etl-replicator:1.2.3');
+
+create or replace function app.update_default_image(new_image_name text)
+returns void
+language plpgsql
+as $$
+declare
+  new_image_id bigint;
+begin
+  -- Serialize concurrent default switches using an advisory transaction lock.
+  perform pg_advisory_xact_lock(('app.images'::regclass::oid)::bigint);
+
+  -- Insert if missing; don't set default yet to respect the partial unique index.
+  insert into app.images(name, is_default)
+  values (new_image_name, false)
+  on conflict (name) do nothing
+  returning id into new_image_id;
+
+  -- If the image already existed, fetch its id.
+  if new_image_id is null then
+    select id into new_image_id from app.images where name = new_image_name;
+  end if;
+
+  -- Unset any existing default that is not the requested image.
+  update app.images
+     set is_default = false, updated_at = now()
+   where is_default = true
+     and id <> new_image_id;
+
+  -- Set the requested image as default.
+  update app.images
+     set is_default = true, updated_at = now()
+   where id = new_image_id;
+end
+$$;