- `membership_makes_app_appear_in_members_app_list` previously seeded
the membership via the repo helper; switch to the public POST
endpoint so the test actually exercises the full HTTP round-trip
the dashboard depends on.
- Add `add_member_with_missing_user_id_is_rejected` to pin the
Axum-JsonRejection 4xx contract on malformed POST bodies.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>