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/worker.go

@@ -221,8 +221,7 @@ func (w *Worker) processJob(job *Job) {
 	}
 	w.mu.Unlock()
 
-	now := time.Now()
-	job.StartedAt = &now
+	job.StartedAt = new(time.Now())
 
 	w.mu.Lock()
 	if result, exists := w.results[job.ID]; exists {
@@ -254,8 +253,7 @@ func (w *Worker) processJob(job *Job) {
 		err = fmt.Errorf("unknown job type: %s", job.Type)
 	}
 
-	completedAt := time.Now()
-	job.CompletedAt = &completedAt
+	job.CompletedAt = new(time.Now())
 	job.Error = err
 	job.Result = result