feat(photos): add square medium variant for sharp grid thumbnails

backend/migrations/000011_photo_medium_variant.down.sql

@@ -0,0 +1,5 @@
+-- Drop medium bindings before narrowing the role constraint again.
+DELETE FROM photo_objects WHERE role = 'medium';
+ALTER TABLE photo_objects DROP CONSTRAINT IF EXISTS photo_objects_role_check;
+ALTER TABLE photo_objects ADD CONSTRAINT photo_objects_role_check
+    CHECK (role IN ('original', 'thumb', 'preview'));