* 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
