feat(db): promote CatalogueCategoryProperty.uid INDEX to CONSTRAINT

[JanSmrcka]

Context

PR #416 added a range INDEX on `CatalogueCategoryProperty.uid` (migration `20260511120000_add_catalogue_category_property_uid_index`) instead of a uniqueness CONSTRAINT, to avoid dirty-state risk if duplicate UIDs exist in prod data.

Every other identifier in the schema (`CatalogueItem.uid`, `Unit.uid`, `Manufacturer.name`, `User.uid`, `Role.uid`, `CatalogueCategoryPropertyType.uid`) uses a CONSTRAINT. CatalogueCategoryProperty is the inconsistent one.

Steps

1. Audit prod for duplicates: `MATCH (p:CatalogueCategoryProperty) WITH p.uid AS u, count(*) AS c WHERE c > 1 RETURN u, c`.
2. If clean → add migration `CREATE CONSTRAINT CatalogueCategoryProperty_uid_unique IF NOT EXISTS FOR (p:CatalogueCategoryProperty) REQUIRE p.uid IS UNIQUE` + drop the bare INDEX (constraint creates its own).
3. If duplicates exist → data-cleanup migration first, then constraint.

Why

Closes the semantic gap, prevents accidental duplicate-UID creation in code paths that don't go through MERGE.