Add Postgres-compatible request logging (#127)

Add Postgres-compatible request logging (#127)

Abstract dashboard request logging behind Kysely so it runs on either
SQLite (default) or Postgres, selected via logging.driver. The schema and
migrations are shared across both backends, existing SQLite databases are
baselined and upgraded in place, and Logger reads/writes are async with
numeric coercion for Postgres aggregates. Includes unit, legacy-upgrade,
and live-Postgres integration tests plus updated config/docs.