Add the opt-in environmental-context store: per-day EnvironmentContext
observations (temperature, daylight, sunshine, precipitation, pressure,
humidity, cloud, weather code) keyed by user + day and location, plus the
manual EnvironmentTravelLocation overrides and the coarse home-location
columns on the user. Location is city-level by design; the weather model
needs no finer than ~10-25 km and no GPS track is ever stored.
