List premium products

Server-authoritative pricing for the premium SKUs (Supporter / Ad-free / two generation-credit packs). Every product carries its USD price and the sats an offer would mint for it now; pricing says which of the two the server prices from (usd: sats computed from the USD price at the current BTC/USD rate; sats: the legacy sats prices). 503 PRICES_UNAVAILABLE when a sats price cannot be computed (no usable rate). enabled:false = purchases temporarily unavailable — the /premium page renders its coming-soon state from this flag; already-purchased benefits keep working regardless of this flag. Edge-cached for 60 seconds.