feat(server/m7-379): extract MusicBrainz IDs from audio tags during scan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/library/mbids.go

@@ -0,0 +1,59 @@
+package library
+
+// MusicBrainz tag keys vary by audio format. dhowden/tag's Raw() map exposes
+// the format-specific entries as-is — so we look up multiple candidates and
+// take the first non-empty match.
+//
+// References:
+//   - ID3v2 (MP3):       TXXX frames keyed by description.
+//   - Vorbis (FLAC/OGG): all-caps comment names.
+//   - MP4 (M4A):         iTunes freeform atoms ("----:com.apple.iTunes:...").
+
+var albumMBIDKeys = []string{
+	"TXXX:MusicBrainz Album Id",
+	"MUSICBRAINZ_ALBUMID",
+	"----:com.apple.iTunes:MusicBrainz Album Id",
+}
+
+var artistMBIDKeys = []string{
+	"TXXX:MusicBrainz Artist Id",
+	"MUSICBRAINZ_ARTISTID",
+	"----:com.apple.iTunes:MusicBrainz Artist Id",
+}
+
+// extractMBIDs reads MusicBrainz album and artist IDs from a tag.Metadata
+// Raw() map. Returns ("", "") when the keys are absent. Multi-value keys
+// (some formats list multiple artist IDs for collaborations) take only the
+// first value — Minstrel uses the primary release artist for MBCAA lookup.
+func extractMBIDs(raw map[string]interface{}) (albumMBID, artistMBID string) {
+	albumMBID = firstStringValue(raw, albumMBIDKeys)
+	artistMBID = firstStringValue(raw, artistMBIDKeys)
+	return
+}
+
+func firstStringValue(raw map[string]interface{}, keys []string) string {
+	for _, k := range keys {
+		v, ok := raw[k]
+		if !ok || v == nil {
+			continue
+		}
+		switch x := v.(type) {
+		case string:
+			if x != "" {
+				return x
+			}
+		case []string:
+			if len(x) > 0 && x[0] != "" {
+				return x[0]
+			}
+		case []interface{}:
+			if len(x) == 0 {
+				continue
+			}
+			if s, ok := x[0].(string); ok && s != "" {
+				return s
+			}
+		}
+	}
+	return ""
+}