Clean up orphan images

migrations/20261012120000_image_upload_orphans.sql

@@ -0,0 +1,4 @@
+ALTER TABLE image_uploads
+    -- UTC time at which cleanup first observed that this upload had no references.
+    ADD COLUMN orphaned_at DATETIME(6) NULL DEFAULT NULL,
+    ADD INDEX image_uploads_orphaned_at (orphaned_at);