OK: column `files.file_size_bytes` already present, skipping.
OK: column `files.client_modified_at` already present, skipping.
OK: column `files.unreadable_reason` already present, skipping.

Migration complete. Existing rows have NULL for the new columns until
they're next rescanned (upsert on re-process will fill them in) --
nothing needs to be backfilled manually.