Real 24h price change derived from candle store

[ozpool]

Problem

pseudoChange(market) on /markets returns ((hashBase(market) % 1601) - 800) / 100 — a deterministic but completely fake number in [-8%, +8%].

MarketHeader's 24h change works correctly against the local anchor price (first observed value after page load), but that's "change since you opened the tab", not 24h.

Proposal

Once #152 ships, MarketInfo can carry a change24hPct field computed at REST read time:

fn change_24h_pct(market_id: &str) -> Decimal {
    let candles = self.candles_24h(market_id);
    let open = candles.first().map(|c| c.open).unwrap_or_default();
    let close = candles.last().map(|c| c.close).unwrap_or_default();
    if open.is_zero() { return dec!(0); }
    ((close - open) / open) * dec!(100)
}

FE: drop pseudoChange everywhere, read market.change24hPct.

Acceptance

• /markets table 24h change column shows real numbers.
• MarketHeader's % matches across page reloads instead of resetting.

Depends on

#152