Replace watch_new_dir (which spawned a redundant background inotifywait
per directory and silently lost files) with a synchronous scan_new_dir
that immediately finds and adds all files. The main recursive watcher
already handles future events in new subdirectories.

The grep patterns (',.*S' and '-v ,.*S') matched Dovecot's size
metadata (,S=7520) instead of the actual Maildir Seen flag after :2,.
This prevented read status from propagating from notmuch to IMAP.

Replace grep-based filtering with case statements that check the
flags field after :2, correctly.