add_trip_member resolves an existing account by username or email, so a
travelling companion who never signed up could be put on a trip in the
planner and not through an assistant. A guest is assignable to budget
splits, packing, to-dos and day participants like any member, so leaving
them out of an import loses who was actually there.
Mirrors the owner-only /api/trips/:id/guests routes, including the
trip-scoped guard that keeps one trip's owner away from another trip's
guest and off a real member's user row.
Maurice
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