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

backend/migrations/000011_photo_medium_variant.up.sql

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+-- The grid serves a square medium variant between thumb and preview.
+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', 'medium', 'preview'));
+
+-- Regenerate variants for existing photos so they gain the medium variant and a
+-- square thumb. The worker rebinds roles; superseded variants are reclaimed by
+-- the reference-counting collector.
+INSERT INTO variant_jobs (photo_id)
+SELECT id FROM photos
+ON CONFLICT (photo_id) DO NOTHING;