#942's normalize_spotify_oauth_config trimmed whitespace/quotes (good — those can't be part of a
real credential) but ALSO rstrip("/")'d the redirect_uri. that's unsafe: Spotify matches the
redirect URI EXACTLY against the app's dashboard registration, and a trailing slash is a
legitimate part of a URI. stripping it would silently break anyone who registered '…/callback/'
(we'd send '…/callback' → INVALID_CLIENT: Invalid redirect URI) — trading one failure mode for a
sneakier one the user can't diagnose (SoulSync no longer sends what they typed).
drop the rstrip; keep the whitespace/quote trim. the value is now preserved verbatim apart from
unambiguous paste garbage. flipped the test that asserted the strip to assert the slash is kept
(and that whitespace/quotes around it are still trimmed), + a dedicated regression guard.
the #942 integration test mocks normalize, so it's unaffected. 262 spotify/oauth tests green.
credit: builds on HellRa1SeR's #942.
