When no cached token exists, spotipy's auth probe starts an interactive
OAuth flow that binds 127.0.0.1:<redirect_port> inside the container.
This either steals Flask's port 8008 (crash loop) or binds loopback-only
on 8888 (unreachable from Docker host — 'connection reset by peer').
Now checks for a cached token before probing. If none exists, returns
False immediately so users authenticate via the SoulSync web UI instead.
No behavior change for already-authenticated users.
Fixes#269