fix(bitcoin): round send fee_rate to 3 decimals

[TaprootFreak]

Summary

The BTC payout pipeline silently stalled today (2026-05-20) for ~80 minutes — every sendmany/send RPC was rejected by Bitcoin Core with Invalid amount (error code -3). A 120k EUR buy-crypto (and several smaller ones) sat in Created until the stuck batch was manually cleared.

Root cause

Bitcoin Core's send / sendmany RPC parses the fee_rate argument with decimals=3 (source):

// Fee rates in sat/vB cannot represent more than 3 significant digits.
cc.m_feerate = CFeeRate{AmountFromValue(fee_rate, /*decimals=*/3)};

ParseFixedPoint returns false when the parsed value has more than 3 decimal places, which AmountFromValue re-throws as "Invalid amount".

Our fee-rate computation chain produces values with floating-point artifacts:

estimatesmartfee(1) -> 0.00001935 BTC/kvB
                    * 100000 = 1.9349999999999998   (binary64 imprecision)
                    * 2.0    = 3.8699999999999997   (cpfpFeeMultiplier)
JSON.stringify       = "3.8699999999999997"          (16 decimals)
Bitcoin Core         -> "Invalid amount"

Reproduced against the live BTC output node:

$ bitcoin-cli -named send outputs='[...]' fee_rate=3.8699999999999997 ...
error code: -3
error message: Invalid amount

$ bitcoin-cli -named send outputs='[...]' fee_rate=3.87 ...
   (accepted; fails later with "Insufficient funds" — expected, wallet was drained)

This also explains:

• Only Bitcoin was affected (other chains don't go through rpc.send with a sat/vB fee_rate).
• Recovery was non-deterministic — the loop unstuck itself when estimatesmartfee happened to return a value that survived the multiplication without artifacts.

Fix

Round the computed sat/vB fee rate to 3 decimal places in BitcoinBasedFeeService.getSendFeeRate(), which is the single source consumed by PayoutBitcoinService.sendUtxoToMany and the dex Bitcoin strategies.

Test plan

• Added unit test in bitcoin-fee.service.spec.ts covering the exact floating-point case (1.935 * 2 -> 3.87, not 3.8699999999999997).
• npm run lint, npm run format:check, npm run build, full Bitcoin fee test suite: all green locally.
• Verify on DEV that BTC payouts continue to broadcast normally after deploy.