feat(api): library sources, scanner and catalog (M2)

- schema for sources, path patterns, media files, artists, albums,
  tracks, genres (migration 00003)
- path patterns (RE2 with named groups, skip), merge with tags by
  source priority, fallbacks from the file name and folder, name
  normalization and multi-artist split
- scanner: walk, skip unchanged files by size and mtime, ffprobe in 8
  workers with a single catalog writer, renames by content hash keep
  the track, duplicates fold into one track with the best file as
  primary, missing files are kept 30 days
- storage-agent mounts NFS with validated server, export and an option
  whitelist, no shell; test mount counts audio files
- admin API: sources CRUD, connection test, patterns with preview,
  scan, library status, file errors, host folder picker; scheduled
  scans and health checks; mounts are restored on start
- catalog API: artist, artist albums, album with tracks, track
- api image is alpine with ffmpeg for ffprobe

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/library/meta/normalize.go

@@ -0,0 +1,112 @@
+// Package meta — метаданные треков: нормализация имён, шаблоны путей, слияние с тегами
+// (docs/library-and-storage.md).
+package meta
+
+import (
+	"strings"
+	"unicode"
+
+	"golang.org/x/text/unicode/norm"
+)
+
+// Clean приводит значение к виду для показа: NFC, `_` → пробел, без лишних пробелов.
+func Clean(s string) string {
+	s = norm.NFC.String(s)
+	s = strings.ReplaceAll(s, "_", " ")
+	return strings.Join(strings.Fields(s), " ")
+}
+
+// Key — ключ сопоставления (name_norm): нижний регистр, ё → е, без пунктуации, пробелы схлопнуты.
+// «The Beatles», «the  beatles!» и «THE BEATLES» дают один ключ.
+func Key(s string) string {
+	s = norm.NFKC.String(s)
+	var b strings.Builder
+	b.Grow(len(s))
+	space := false
+	for _, r := range strings.ToLower(s) {
+		switch {
+		case r == 'ё':
+			r = 'е'
+		case unicode.IsPunct(r) || unicode.IsSymbol(r) || unicode.IsSpace(r) || r == '_':
+			space = b.Len() > 0
+			continue
+		}
+		if space {
+			b.WriteByte(' ')
+			space = false
+		}
+		b.WriteRune(r)
+	}
+	return b.String()
+}
+
+// SortName — имя для сортировки исполнителей: без ведущего артикля «The ».
+func SortName(name string) string {
+	if rest, ok := strings.CutPrefix(name, "The "); ok && rest != "" {
+		return rest
+	}
+	return name
+}
+
+// Разделители нескольких исполнителей в одном поле, кроме « & ».
+var artistSeparators = []string{";", " / ", " feat. ", " feat ", " ft. ", " Feat. ", " Ft. ", " featuring "}
+
+// ArtistCredit — исполнитель трека и его роль.
+type ArtistCredit struct {
+	Name string
+	Role string // main | feat
+}
+
+// SplitArtists разбирает поле исполнителя на отдельных. « & » делит имя, только если составного
+// исполнителя нет в библиотеке (known), — иначе «Simon & Garfunkel» распался бы на двоих.
+// Всё после feat./ft. — приглашённые исполнители.
+func SplitArtists(s string, known func(key string) bool) []ArtistCredit {
+	s = Clean(s)
+	if s == "" {
+		return nil
+	}
+	var out []ArtistCredit
+	seen := map[string]bool{}
+	add := func(name, role string) {
+		name = Clean(name)
+		k := Key(name)
+		if k == "" || seen[k] {
+			return
+		}
+		seen[k] = true
+		out = append(out, ArtistCredit{Name: name, Role: role})
+	}
+
+	main, feat := s, ""
+	for _, sep := range []string{" feat. ", " feat ", " ft. ", " Feat. ", " Ft. ", " featuring "} {
+		if i := strings.Index(main, sep); i >= 0 {
+			main, feat = main[:i], main[i+len(sep):]
+			break
+		}
+	}
+	for _, part := range splitAny(main, artistSeparators) {
+		if strings.Contains(part, " & ") && (known == nil || !known(Key(part))) {
+			for _, p := range strings.Split(part, " & ") {
+				add(p, "main")
+			}
+			continue
+		}
+		add(part, "main")
+	}
+	for _, part := range splitAny(feat, append(artistSeparators, " & ", ", ")) {
+		add(part, "feat")
+	}
+	return out
+}
+
+func splitAny(s string, seps []string) []string {
+	parts := []string{s}
+	for _, sep := range seps {
+		var next []string
+		for _, p := range parts {
+			next = append(next, strings.Split(p, sep)...)
+		}
+		parts = next
+	}
+	return parts
+}