A transient ping failure (network blip, Navidrome busy mid-scan) makes
_setup_client null out the configured creds, and _connection_attempted then
latches the client "disconnected" — so is_connected() returned False forever until
the user hit the manual Test button to re-read config. That's the reported
"disconnects every 5-10 min, reconnects instantly on Test."
Fix: ensure_connection no longer latches on a failed attempt — once a short
throttle (_RECONNECT_THROTTLE_S = 20s) elapses it re-attempts, and is_connected()
triggers that retry whenever it's currently disconnected. So a blip recovers on its
own within the next status check, no manual reconnect. The throttle prevents ping
storms when Navidrome is genuinely down.
Tests: transient failure self-heals after the throttle (and doesn't re-ping within
it); a connected client never re-pings; first connect attempts once. 115 navidrome/
media-server tests green.
