docs(api): document intentional audioContentType divergence from subsonic

internal/api/library_fixtures_test.go

@@ -107,6 +107,7 @@ func seedTrackWithFile(t *testing.T, pool *pgxpool.Pool, albumID, artistID pgtyp
 		ArtistID:    artistID,
 		TrackNumber: &one,
 		DiscNumber:  nil,
+		// DurationMs: arbitrary non-zero; tests don't assert on it.
 		DurationMs:  int32(len(fileBody)),
 		FilePath:    path,
 		FileSize:    int64(len(fileBody)),

internal/api/media.go

@@ -57,6 +57,12 @@ func resolveAlbumCoverPath(ctx context.Context, q *dbq.Queries, album dbq.Album)
 // ogg, opus, m4a, aac, wav) to a MIME type for the Content-Type header.
 // Unknown formats fall back to octet-stream so the browser downloads them
 // rather than attempting to decode.
+//
+// Divergences from internal/subsonic/types.go's contentTypeForFormat are
+// intentional: opus→audio/ogg (library .opus files are Ogg-encapsulated, so
+// this matches real library contents), aac→audio/aac (raw AAC is ADTS, not
+// MP4, so audio/mp4 would mislead codec sniffers), and there is no "oga" case
+// (we don't record that format). Don't "fix" these to match subsonic.
 func audioContentType(format string) string {
 	switch strings.ToLower(format) {
 	case "mp3":