A large concurrent burst of getUserById calls during the tunnels-list
email enrichment self-inflicts an upstream throttle (truncated/empty
bodies) that even the per-call retry can't fully escape, intermittently
rendering owner_email as '—'. Add mapWithConcurrency and resolve owner
emails at most a few at a time so each lookup stays inside the throttle
allowance; retry + null fallback preserved.
Gerhard Scheikl