move UTXO lockUnspent from build() to sendOnchain()

build() is called from both /payments (the actual send path) and /fee
(the fee-estimation endpoint). Locking UTXOs in build() meant that every
fee quote was reserving inputs that never got broadcast, slowly leaking
UTXO locks until a bitcoind restart cleared them — eventually starving
the wallet of selectable inputs even when balance was healthy.

The lock belongs at the point where we're actually about to broadcast,
which is sendOnchain. Released in the catch path on broadcast failure;
on success, the inputs become spent and the lock is moot.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

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