fix(scanner): split comma-joined ABS sidecar genres into separate tags

Audiobookshelf-style exports join genres into one comma-string
(["Epic, Action & Adventure, Military, Sword & Sorcery"]). The
smart-genre picker already split candidates for the genre column,
but the tags assembly used the raw unsplit array — so the whole
string landed as ONE element of media_items.tags and rendered as a
single tag everywhere.

Use pickSmartGenre's split candidates for tags too, exactly as the
function's own comment documents ('split parts included stay in
tags'). Commas inside a single subject heading remain legal within
one tag (EPUB dc:subject path unchanged).

internal/services/media_scanner_metadata_test.go

@@ -8,6 +8,7 @@ import (
 	"image/jpeg"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -196,6 +197,30 @@ func TestExtractAudiobookshelfSidecar(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "joined genre string splits into separate tags (ABS export)",
+			json: `{"title": "Joined Genres", "genres": ["Epic, Action & Adventure, Military, Sword & Sorcery"]}`,
+			validate: func(t *testing.T, m *MediaMetadata) {
+				want := []string{"Epic", "Action & Adventure", "Military", "Sword & Sorcery"}
+				if len(m.Tags) != len(want) {
+					t.Fatalf("Tags = %v, want %v", m.Tags, want)
+				}
+				for i, w := range want {
+					if m.Tags[i] != w {
+						t.Errorf("Tags[%d] = %q, want %q", i, m.Tags[i], w)
+					}
+				}
+				inGenre := false
+				for _, w := range want {
+					if strings.EqualFold(m.Genre, w) {
+						inGenre = true
+					}
+				}
+				if !inGenre || m.Genre == "" {
+					t.Errorf("Genre = %q, want a case-insensitive match of %v", m.Genre, want)
+				}
+			},
+		},
 		{
 			name: "sparse sidecar (real-world Audiobookshelf export)",
 			json: `{"title": "Sparse (1234)", "authors": ["X"], "tags": [], "description": null}`,