The root logger previously had no handler at all, so every module's
logger.info() call (user creation, claims, password resets, ...) was
silently dropped, not just unviewable. Adds a RotatingFileHandler
writing into the existing /data volume so log content also survives
container restarts/redeploys, plus /admin/logs (tail + line-count
picker + full-file download) alongside the existing Users & Frames
admin page.
