Fix flaky tests caused by colliding factory slugs

[Cannonb4ll]

Problem

ViewItemTest > A user with admin access can render a private item page that has no associated project failed now and then in CI with a 302 instead of 200.

The factories for Item, Project, Board and Changelog generated slugs with $this->faker->word, which picks from a list of about 180 lorem words. ViewItemTest's beforeEach already creates an item that belongs to a project. When the test's own item got the same slug, ItemController::show (where('slug', ...)->firstOrFail()) resolved the project item and redirected to its project page.

Reproduced locally by running the test in a loop:

{"id":1,"slug":"dolorem","project_id":1}
{"id":2,"slug":"dolorem","project_id":null}
→ 302 to /projects/recusandae/items/dolorem

Fix

Use $this->faker->unique()->slug() for the slug in all four factories.

Verification

• Before: the test failed after 14–137 runs
• After: 300 runs of ViewItemTest with 0 failures
• Full suite passes (287 tests)