Add `User.homeSince` (nullable effective-from instant) and a `DEVICE` value to
the location-source enum (migration 0219). `homeSince` records when the home
location became effective so day resolution can attribute a past day to the
home only on/after that instant, rather than to the current home for every past
day. Existing accounts with a home get `homeSince` stamped to the migration
instant; their deep past is filled by explicit location periods instead.
`DEVICE` is reserved for a future client-supplied coarse per-day location: the
enum value and the wire contract accept it now so the resolver can rank it
later without rework. Nothing writes it server-side yet.
