Soft-delete a gate (moves to trash for 30 days)

Use when: caller wants to delete a gate (soft-delete; stays in trash for 30 days before permanent removal)

Soft-deletes a gate — it moves to the trash for a 30-day retention window before the trash-cleanup worker hard-deletes it. Rejected with 409 if the gate has active dependencies (sub-gate attachments, running sessions); pass ?force=true to override (use with caution — the trash-cleanup worker handles related cleanup downstream).