fix(scanner): always attempt cover extraction for EPUBs and relax manga detection

Two changes to EPUB metadata extraction:

1. Restructure extractMetadata so that fixed-layout detection and cover
   extraction always run for EPUBs, even when extractEPUBMetadata returns
   an error. Previously, a partial failure from the EPUB parser would skip
   cover and format detection entirely, leaving books without covers.

2. Remove the language restriction (ja/jpn) from manga reading direction
   detection. Manga tagged with 'manga' should default to RTL regardless
   of the language metadata, since the tag is an explicit signal from the
   user or metadata source.

internal/services/media_scanner.go

@@ -953,7 +953,7 @@ func determineReadingDirection(comicInfo *ComicInfo) string {
 	if strings.Contains(tags, "webtoon") || strings.Contains(tags, "manhwa") {
 		return "vertical" // Korean/Chinese webcomics
 	}
-	if strings.Contains(tags, "manga") && (lang == "ja" || lang == "jpn") {
+	if strings.Contains(tags, "manga") {
 		return "rtl" // Japanese manga
 	}
 
@@ -1077,8 +1077,14 @@ func (s *MediaScanner) extractMetadata(path string) (*MediaMetadata, error) {
 
 	switch ext {
 	case ".epub":
-		metadata, err := s.extractEPUBMetadata(path)
+		metadata := &MediaMetadata{}
-		if err != nil {
+		result, err := s.extractEPUBMetadata(path)
+		if err == nil {
+			return result, nil
+		}
+		if result != nil {
+			metadata = result
+		}
 		// Enhanced format detection for EPUBs
 		isFixedLayout, detectErr := s.DetectFixedLayoutEPUB(path)
 		if detectErr == nil && isFixedLayout {
@@ -1104,8 +1110,6 @@ func (s *MediaScanner) extractMetadata(path string) (*MediaMetadata, error) {
 			}
 		}
 		return metadata, nil
-		}
-		return metadata, nil
 	case ".pdf":
 		return s.extractPDFMetadata(path)
 	default: