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_test.go

@@ -0,0 +1,69 @@
+package library
+
+import "testing"
+
+func TestExtractMBIDs_ID3v2(t *testing.T) {
+	raw := map[string]interface{}{
+		"TXXX:MusicBrainz Album Id":  "abc-123",
+		"TXXX:MusicBrainz Artist Id": "def-456",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc-123" || ar != "def-456" {
+		t.Errorf("got (%q, %q), want (\"abc-123\", \"def-456\")", a, ar)
+	}
+}
+
+func TestExtractMBIDs_Vorbis(t *testing.T) {
+	raw := map[string]interface{}{
+		"MUSICBRAINZ_ALBUMID":  "abc",
+		"MUSICBRAINZ_ARTISTID": "def",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc" || ar != "def" {
+		t.Errorf("got (%q, %q)", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MP4(t *testing.T) {
+	raw := map[string]interface{}{
+		"----:com.apple.iTunes:MusicBrainz Album Id":  "abc",
+		"----:com.apple.iTunes:MusicBrainz Artist Id": "def",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc" || ar != "def" {
+		t.Errorf("got (%q, %q)", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MissingKeysReturnEmpty(t *testing.T) {
+	a, ar := extractMBIDs(map[string]interface{}{})
+	if a != "" || ar != "" {
+		t.Errorf("got (%q, %q), want empty", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MultiValueArtistTakesFirst(t *testing.T) {
+	// Some FLAC tags list collaborators as multi-value MUSICBRAINZ_ARTISTID.
+	// Minstrel uses the primary release artist; first wins.
+	raw := map[string]interface{}{
+		"MUSICBRAINZ_ARTISTID": []string{"primary-mbid", "collab-mbid"},
+	}
+	_, ar := extractMBIDs(raw)
+	if ar != "primary-mbid" {
+		t.Errorf("got %q, want %q", ar, "primary-mbid")
+	}
+}
+
+func TestExtractMBIDs_NilEntryIgnored(t *testing.T) {
+	raw := map[string]interface{}{
+		"TXXX:MusicBrainz Album Id":  nil,
+		"TXXX:MusicBrainz Artist Id": "real-id",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "" {
+		t.Errorf("nil album entry should be ignored, got %q", a)
+	}
+	if ar != "real-id" {
+		t.Errorf("artist = %q", ar)
+	}
+}