refactor(services): replace temporary variable pointer pattern with new() builtin

Simplify pointer creation in media scanner validation messages and worker
job timestamps by using inline new() instead of local variable + address-of.

In media_scanner.go this cleans up three validation error message returns
(manga/comics library format checks). In worker.go it simplifies StartedAt
and CompletedAt timestamp assignments.

internal/services/media_scanner.go

@@ -1297,13 +1297,12 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Must be fixed-layout or comic archive
 		if mediaItem.FormatGroup != "fixed_layout" &&
 			mediaItem.FormatGroup != "comic_archive" {
-			msg := fmt.Sprintf(
+			return new(fmt.Sprintf(
 				"EPUB file '%s' is reflowable (text-based), not fixed-layout (image-based). "+
 					"Manga library only accepts fixed-layout EPUBs, CBZ, CBR, or image files. "+
 					"Consider moving this file to an ebooks library.",
 				mediaItem.Title,
-			)
-			return &msg
+			))
 		}
 
 		// Set manga-specific flags for fixed-layout EPUBs
@@ -1328,12 +1327,11 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Accept comic archives and fixed-layout
 		if mediaItem.FormatGroup != "comic_archive" &&
 			mediaItem.FormatGroup != "fixed_layout" {
-			msg := fmt.Sprintf(
+			return new(fmt.Sprintf(
 				"File '%s' is not a comic archive format. "+
 					"Comics library only accepts CBZ, CBR, CB7, CBT, PDF, or fixed-layout EPUBs.",
 				mediaItem.Title,
-			)
-			return &msg
+			))
 		}
 	}
 
@@ -1342,12 +1340,11 @@ func (s *MediaScanner) ValidateMediaItemForLibrary(
 		// Flag manga for potential reorganization (info level)
 		if mediaItem.FormatGroup == "fixed_layout" &&
 			(!mediaItem.MangaType.Valid || mediaItem.MangaType.String == "yes" || mediaItem.MangaType.String == "yes_and_right_to_left") {
-			msg := fmt.Sprintf(
+			return new(fmt.Sprintf(
 				"File '%s' appears to be manga (fixed-layout with images). "+
 					"Consider moving to a manga or comics library for better organization.",
 				mediaItem.Title,
-			)
-			return &msg
+			))
 		}
 	}