feat: add a unique constraint for fluxstd

[monodera]

Unique constraint is set by (obj_id, input_catalog_id, version).

Before applying the modification by alembic, I removed duplicate entries with the following query:

DELETE FROM fluxstd
WHERE fluxstd_id IN (
  SELECT b.fluxstd_id
  FROM fluxstd a
  JOIN fluxstd b ON a.obj_id = b.obj_id
                AND a.input_catalog_id = b.input_catalog_id
                AND a.version = b.version
                AND a.fluxstd_id < b.fluxstd_id
);

DELETE 99887