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.

transaction_tracker/__init__.py

@@ -40,9 +40,17 @@ class TransactionTracker:
         self.logger.debug(f"Transaction {transaction_id} marked as processed")
 
     def is_processed(self, transaction_id):
-        """Check if transaction was already processed."""
+        """Check if transaction was already processed and its record has not expired."""
         record_path = os.path.join(self.storage_dir, f"{transaction_id}.json")
-        return os.path.exists(record_path)
+        if not os.path.exists(record_path):
+            return False
+        try:
+            with open(record_path, "r") as f:
+                record = json.load(f)
+            expires_at = datetime.fromisoformat(record["expires_at"])
+            return datetime.now() < expires_at
+        except (json.JSONDecodeError, KeyError, ValueError, TypeError):
+            return False
 
     def require_unique_transaction(self, id_arg='transaction_id'):
         """