test: add tests for delete profile endpoint

[AFKFelix]

The new DELETE /profiles/:category/:name endpoint is not covered by the existing E2E suite (tests/e2e/profiles.spec.ts currently covers POST/GET only). Add tests for successful deletion (204) and for expected error cases (e.g., invalid category/name, and deleting a non-existent profile with the current error semantics).

Originally posted by @Copilot in #39 (comment)