fix(tracker): is_processed now checks expires_at, not just file existence 🐛

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.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "transaction-tracker"
-version = "0.1.0"
+version = "0.1.3"
 description = ""
 authors = [
     {name = "Aaron Guise",email = "aaron@guise.net.nz"}