Fix flaky categories_spec destroy test

[maebeale]

Closes #1477

What is the goal of this PR and why is this important?

• Fix an intermittently failing test: Categories as an admin DELETE destroy destroys category
• The test fails with expected Category.count to have changed by -1, but was changed by 0

How did you approach the change?

• Root cause: The unique index (category_type_id, position) on the categories table is incompatible with the positioning gem. During destroy/update, the gem temporarily sets position to 0 via update_all. If another record in the same scope already has position 0 (from an interrupted prior operation), MySQL raises ActiveRecord::RecordNotUnique, causing destroy! to fail
• The unique index was added in Add drag-and-drop positioning for categories scoped by category type #712 but the positioning gem manages uniqueness itself through row-level locking — the DB constraint is redundant and fights with the gem's intermediate states
• Fix: Migration to replace the unique composite index with a non-unique composite index (preserves query performance, removes the constraint that conflicts with the gem)

Anything else to add?

• Reproduced by manually creating a position-0 record and confirming destroy! raises ActiveRecord::RecordNotUnique
• The positioning gem uses position 0 and negative numbers as intermediate values during reordering — this is by design and documented in the gem's source

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