A user's spaces are each seeded with the same default category names, so the
per-user unique indexes on categories, labels and saved_filters collided the
moment a second space was created (500 on space creation). Rescope those
uniques to space_id (adding a plain user_id index first so the FK keeps its
index), teach the category duplicate-name handler the new index name, and
harden the creation test to seed the personal space first so it reproduces the
collision.
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