findOrCreate used tagRepository.findByNameIgnoreCase, which returns
Optional<Tag> and threw NonUniqueResultException whenever two tags
collided case-insensitively (a canonical parent and its same-named
lowercase child). Every document carrying such a tag became un-editable:
any save re-resolves the whole tag set by name and blew up with a 500.
Replace the throwing lookup with exact-case-first resolution: findByName
(exact) → findAllByNameIgnoreCase (lowest-id, deterministic, never
throws) → create. Delete findByNameIgnoreCase so the throwing call can't
be reintroduced. Case collisions are valid tree nodes — no migration, no
unique(lower(name)) constraint.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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