Replace unbounded HashSet<String> with a BoundedSeenSet that evicts
the oldest message IDs (FIFO) when the 100k capacity is reached. This
prevents memory growth proportional to email volume over the process
lifetime, capping the set at ~100k entries regardless of runtime.
Closes#349
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>