Implement the OpenSubsonic transcoding extension (getTranscodeDecision/getTranscodeStream)

Adds a real capability-matching decision engine: getTranscodeDecision
compares the source file's actual scanned format/channels against the
client's declared directPlayProfiles first, and only when nothing
matches does it search transcodingProfiles for a codec VibeSonic's
ffmpeg pipeline can actually encode (mp3/opus/aac), applying any
codecProfile limitations (bitrate/channel required constraints) the
client listed. canTranscode is never set true for a codec the server
has no encoder for, and canDirectPlay is never set true without a
matching client profile.

getTranscodeStream takes the opaque transcodeParams token the
decision returned (never re-derived) plus a real time offset and
streams actual transcoded audio via the existing seek-capable
transcoding.Manager.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/subsonic/opensubsonic_core_test.go

@@ -55,7 +55,7 @@ func TestGetOpenSubsonicExtensions(t *testing.T) {
 	// set once Phase 3 is done.
 	for _, want := range []string{
 		"apiKeyAuthentication", "formPost", "getPodcastEpisode", "topSongsByArtistId", "transcodeOffset",
-		"indexBasedQueue", "playbackReport", "songLyrics",
+		"indexBasedQueue", "playbackReport", "songLyrics", "transcoding",
 	} {
 		if !names[want] {
 			t.Errorf("extension %q not advertised: %v", want, names)