fix(scanner): pass filename hint to gotaglib's OpenStream for format detection (#5012)

* fix: split reflex -R flags to preserve directory exclusion optimization

Combining the _test.go exclusion pattern (which uses $) into the same -R
regex as the directory prefixes (^ui, ^data, ^db/migrations) disabled
reflex's ExcludePrefix optimization. Reflex disables prefix-based
directory skipping when the regex AST contains $, \z, or \b operators,
causing it to traverse into ui/node_modules and hit "too many open files".

Splitting into two separate -R flags fixes this: the directory prefix
regex remains $-free so ExcludePrefix works, while the _test.go pattern
gets its own flag where the $ anchor doesn't affect directory skipping.

* fix(gotaglib): pass filename hint to OpenStream for format detection

OpenStream relies on content-sniffing when no filename is provided,
which fails for some files (e.g. OPUS). Pass the filename via the new
WithFilename option so TagLib can use the file extension as a hint.

Also adds an OPUS test fixture and test entry.

Relates to https://github.com/navidrome/navidrome/issues/4604#issuecomment-3868569113, #4998, #5010

adapters/gotaglib/gotaglib.go

@@ -119,7 +119,12 @@ func (e extractor) openFile(filePath string) (f *taglib.File, closeFunc func(),
 		file.Close()
 		return nil, nil, errors.New("file is not seekable")
 	}
-	f, err = taglib.OpenStream(rs, taglib.WithReadStyle(taglib.ReadStyleFast))
+	// WithFilename provides a format detection hint via the file extension,
+	// since OpenStream alone relies on content-sniffing which fails for some files.
+	f, err = taglib.OpenStream(rs,
+		taglib.WithReadStyle(taglib.ReadStyleFast),
+		taglib.WithFilename(filePath),
+	)
 	if err != nil {
 		file.Close()
 		return nil, nil, err