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

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

@@ -0,0 +1,24 @@
+-- SQLite cannot alter a CHECK constraint, so recreate photo_objects with the
+-- extended role set (medium added) and copy the existing bindings across.
+CREATE TABLE photo_objects_new (
+    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', 'medium', 'preview')),
+    PRIMARY KEY (photo_id, role)
+);
+
+INSERT INTO photo_objects_new (photo_id, object_id, role)
+SELECT photo_id, object_id, role FROM photo_objects;
+
+DROP TABLE photo_objects;
+
+ALTER TABLE photo_objects_new RENAME TO photo_objects;
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);
+
+-- Regenerate variants for existing photos so they gain the medium variant and a
+-- square thumb.
+INSERT INTO variant_jobs (photo_id)
+SELECT id FROM photos
+WHERE 1
+ON CONFLICT (photo_id) DO NOTHING;