apiKey (OpenSubsonic) is preferred. t+s uses md5(subsonic_password+salt)
with a constant-time compare. p is disabled by default and gated by
SubsonicConfig.AllowPlaintextPassword; enc:HEX obfuscation decoded.
Auth failures write a Subsonic "failed" envelope so clients don't see
HTTP 401.
Renders one wire format per ?f= parameter; JSON wraps in
{"subsonic-response":...}, XML emits with xmlns, JSONP wraps in callback
or falls back to JSON when callback is empty.
bvandeusen