feat: calculate spread-based fees for Scrypt trades

[TaprootFreak]

Summary

• Scrypt reports feeAmount=0 for all trades, but the actual cost is the spread between execution price and market rate
• Adds automatic spread fee calculation using historical prices via getPriceAt() during exchange TX sync
• Preserves calculated fees across re-syncs (prevents Object.assign from overwriting with DTO's feeAmount=0)

Details

• New calculateSpreadFee() method in ExchangeTxService
• Uses AssetPrice table for historical market rates at exact trade date
• Supports all Scrypt pairs: USDT/CHF, USDT/EUR, BTC/USDT (buy & sell)
• Fee = amount × (executionPrice − marketRate) in quote currency, converted to CHF

After deploy

One-time resync needed for existing ~741 trades:

await exchangeTxService.syncExchanges(new Date('2026-01-19'), ExchangeName.SCRYPT);

Test plan

• Verify new Scrypt trades get feeAmount and feeAmountChf populated
• Verify re-sync does not overwrite calculated fees
• Run backfill for existing trades and verify values match analysis (~13'800 CHF total)
• Verify non-Scrypt exchanges are unaffected