Function ensure_wal_mode 

pub fn ensure_wal_mode(conn: &Connection) -> Result<(), AppError>

Re-asserts PRAGMA journal_mode = WAL after operations that may revert it (notably refinery-driven migrations, which can open internal handles that reset the journal mode in some scenarios). Idempotent and cheap; emits tracing::warn! if WAL fails to engage so degraded behaviour is observable.