TIERS_V2 launch: Physical tier and Realtime Pro

At launch, the ladder moved to Free / Physical / Pro / Realtime Pro. The new Physical tier added real dealer gold prices per country at /v1/physical/{country} (12 countries, 16 sources, with a nullable premium-over-spot field) plus everything the former Basic tier carried. This release also introduced the probability surface and get_surface; both were retired on July 25 and are no longer part of any paid plan (see the 26 July entry). Realtime Pro keeps physical:{country} WebSocket channels. The former Basic tier remains closed to new signups; existing Basic subscribers keep their price, limits, and key unchanged.