The audiobook scanner needs format-level tags (title, artist, album,
date) for media_items metadata; ffprobe already parses them in
ffprobeFormat.Tags but ProbeData previously discarded them. Add
FormatTags map[string]string to ProbeData, populate it in
convertProbeData via a new normalizeFormatTags helper that lowercases
keys and trims values.
Adds a fixture audiobook .m4b with embedded chapters (Intro/Outro) and
format tags, and a test that verifies ProbeFile() returns both
correctly.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
