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

backend/migrations/sqlite/000003_photo_medium_variant.down.sql

@@ -0,0 +1,18 @@
+-- Drop medium bindings and restore the original three-role constraint.
+DELETE FROM photo_objects WHERE role = 'medium';
+
+CREATE TABLE photo_objects_old (
+    photo_id  TEXT NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+    object_id TEXT NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      TEXT NOT NULL CHECK (role IN ('original', 'thumb', 'preview')),
+    PRIMARY KEY (photo_id, role)
+);
+
+INSERT INTO photo_objects_old (photo_id, object_id, role)
+SELECT photo_id, object_id, role FROM photo_objects;
+
+DROP TABLE photo_objects;
+
+ALTER TABLE photo_objects_old RENAME TO photo_objects;
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);