Extract the team-leader assignment logic into a dedicated
``_define_user_id`` helper, shared between ``_compute_user_id`` and
``create``:
- ``_compute_user_id`` now delegates to ``_define_user_id`` instead of
inlining the membership/leader checks.
- ``create`` also assigns ``user_id`` from the default team via the same
helper, so a portal-created ticket gets the team leader assigned.
This centralises the decision (keep current user / fall back to team
leader) in a single, reusable place.
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