chore: Incremental account codes pruning

[sergerad]

Summary

Closes #2444.

The vault and storage prunes are incremental: each block only visits rows that expired since the previous prune. The account-code prune instead re-scanned every account row still valid past the cutoff on every block.

This PR makes the codes prune churn-driven. A code pinned at one prune can only become collectable at a later one if its last referencing row expired between the two cutoffs, so the prune scans just that window for candidates and probes each for a surviving reference — O(churn) per block, like the other two prunes.

Implementation notes:

• Migration 005 adds idx_accounts_code_probe (backs the existence probe) and a single-row prune_progress table recording the cutoff through which code pruning has completed. The marker is updated in the same transaction as the prune, so it is crash-consistent and survives restarts.
• With no marker recorded (first prune after migration, or a fresh database), the previous full-pass query runs once.
• When the cutoff hasn't advanced (e.g. stalled prune_tip), the prune now skips entirely instead of re-scanning the live set.

Changelog

changelog = "none"
reason    = "Internal change only."