test(categories): add co-located unit tests for CategoriesService

[Magrexy]

Summary

Closes #1284
Closes #1285
Closes #1286
Closes #1287

Adds a co-located categories.service.spec.ts with 20 unit tests covering every public method in CategoriesService, replacing the single misplaced assertion in femaleotaku.spec.ts with focused, discoverable coverage.

Why

The only test touching CategoriesService was one it() in backend/src/femaleotaku.spec.ts — a file that bundles unrelated modules into a single describe. That test covers only the create path with depreciation defaults. Not-found paths for findById, update, and delete had zero coverage. The self-parent rejection in update() was also untested. Moving tests to a co-located spec file follows the project convention and makes failures attributable to the exact module.

What was built

backend/src/categories/categories.service.spec.ts:

Test group	What it covers
findAll	Returns all categories; returns empty array when none exist
findById	Returns category by id; throws NotFoundException for missing id
create	Full depreciation defaults; minimal fields; with parentCategoryId; all optional fields
update	Updates existing category; NotFoundException for missing id; BadRequestException for self-parent; preserves depreciation defaults through update cycle; allows changing parentCategoryId
delete	Deletes existing category; NotFoundException for missing id
depreciation defaults	Zero rate; 100% rate; preserves defaults through update cycle

No existing files modified — only additions, so regression risk is low.

Acceptance criteria coverage

• Categories has real, co-located test coverage beyond the single existing test (categories.service.spec.ts — 20 tests across 6 describe groups)

Test plan

• npm test in backend/ — 20/20 passing (all new tests for this feature)

Env vars / Notes

No new environment variables. Tests use lightweight Jest mocks for the TypeORM repository — no database connection required.

[drips-wave]

@Magrexy Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[vercel]

@Magrexy is attempting to deploy a commit to the naijabuz's projects Team on Vercel.

A member of the Team first needs to authorize it.

[yusuftomilola]

No merge conflicts with main. Good co-located unit test coverage for CategoriesService - approving.