Without a transaction, N individual inserts each re-acquire the SQLite
write lock, creating a window where a concurrent sync-log write hits
SQLITE_LOCKED. The whole batch then throws, no checkpoint is saved, and
the inbox ends up with only the emails that inserted before the failure.
Wrapping in one transaction makes the batch atomic and holds the lock
for a single commit instead of N.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
