Previously is_processed() returned True for any record file that existed,
relying entirely on cleanup_expired() (called at __init__) to delete stale
files. Because cleanup runs at container startup — before Akahu transactions
are fetched — any record that expired exactly on that startup would be deleted
and then immediately missed, letting the duplicate through.
Fix: is_processed() reads the expires_at field from the JSON and returns False
if the record has expired, regardless of whether cleanup has run.
Also adds migrate_ttl.py script to retroactively extend expires_at on existing
records that were written under a shorter TTL, and bumps version to 0.1.3.
