v0.3.1: many-to-many contacts:profiles

ProfileRepo.upsert no longer strips a contact key from other profiles when
adding it here. A contact can now belong to any number of profiles
simultaneously. The ScreeningService already walks profiles in list order and
picks the first one whose contactKeys intersect the resolved buckets, so no
service-side change is needed — list order is the tiebreaker.

UI hint on the contact picker updated to reflect the new semantics.

Bump 0.3.0 -> 0.3.1.