Remove errant UNIQUE INDEX on (utxo, txid) that conflicted with the
PRIMARY KEY (token_id, utxo, txid). A single transaction can update
auth chains for multiple tokens, requiring the same (utxo, txid) pair
to exist with different token_ids. The unique index blocked this.
The index was also unnecessary - EXPLAIN QUERY PLAN shows the PRIMARY
KEY index is used for all bcmr_failure lookups.
Add test covering multi-token failure recording.
