fix(coprocessor): persist computed ct128 as local backup

Persist computed ct128 values in a dedicated `ciphertexts128` table as a
failover mechanism when S3 uploads fail and require retry.

Entries are deleted after a successful L2 `add_ciphertext` call. As an
additional safeguard, a periodic GC procedure removes stale entries in
cases where the txn-sender fails to clean up as expected.

Author: goshawk-3