fix(db): suppress clippy::struct_excessive_bools on MailboxSidecarState

The MailboxSidecarState struct now has 4 boolean fields (wal_exists,
shm_exists, journal_exists, live_sidecars) after adding rollback-journal
tracking, which triggers clippy's struct_excessive_bools lint. These
booleans directly model independent filesystem observations and aren't
candidates for enum/bitflag consolidation — each represents a distinct
sidecar file's existence. Suppress the lint with allow attribute.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>