feat(audiobooks): add Author and Narrator PersonKind constants

Discovery audit confirmed item_people.kind is unconstrained smallint
with values 1-6 in use. Reserve 7 = Author, 8 = Narrator for audiobook
people-links written by the upcoming scanner branches.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/models/media.go

@@ -212,6 +212,8 @@ const (
 	PersonKindProducer  PersonKind = 4
 	PersonKindGuestStar PersonKind = 5
 	PersonKindComposer  PersonKind = 6
+	PersonKindAuthor    PersonKind = 7
+	PersonKindNarrator  PersonKind = 8
 )
 
 // String returns the Jellyfin-compatible type string for this PersonKind.
@@ -229,6 +231,10 @@ func (k PersonKind) String() string {
 		return "GuestStar"
 	case PersonKindComposer:
 		return "Composer"
+	case PersonKindAuthor:
+		return "Author"
+	case PersonKindNarrator:
+		return "Narrator"
 	default:
 		return "Unknown"
 	}

internal/models/media_test.go

@@ -0,0 +1,18 @@
+package models
+
+import "testing"
+
+func TestPersonKindAudiobookRoles(t *testing.T) {
+	cases := []struct {
+		kind PersonKind
+		want string
+	}{
+		{PersonKindAuthor, "Author"},
+		{PersonKindNarrator, "Narrator"},
+	}
+	for _, tc := range cases {
+		if got := tc.kind.String(); got != tc.want {
+			t.Errorf("%d.String() = %q, want %q", tc.kind, got, tc.want)
+		}
+	}
+}