fix(minibf): source address-utxo tx_hash from TxoRef, not archive block_data
GET /addresses/{address}/utxos returned an empty tx_hash for any UTxO whose
source block aged out of the archive window: tx_hash was built from a
block_data archive lookup, and on a miss (block_data == None, governed by
max_history) unwrap_or_default yields "". Source it from the UTxO's own
TxoRef (always present) instead.