_syncEmailsImap now stores {uidValidity, lastUid} per mailbox in the
sync_state table after each full sync. Subsequent syncs only fetch
UIDs newer than lastUid (UID N+1:*) and then do an ALL search to
reconcile remote deletions — avoiding a full re-download on every poll.
When UID validity changes the stale local emails are discarded and a
full re-sync is performed automatically.
fake_imap: add uidValidityResult + searchCallQueue so tests can feed
distinct responses to consecutive uidSearchMessages calls.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
