fix:ASS 자막 비활성화 (슬라이드 캡션바와 중복 제거)

- TutorialVideoJob: 자막 생성 제거, subtitlePath에 null 전달
- TutorialAssemblyService: subtitlePath 파라미터 nullable 변경
- VideoAssemblyService: subtitlePath nullable + null일 때 ass 필터 생략

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Services/Video/TutorialAssemblyService.php

@@ -29,7 +29,7 @@ public function assembleFromImages(
         array $durations,
         ?string $narrationPath,
         ?string $bgmPath,
-        string $subtitlePath,
+        ?string $subtitlePath,
         string $outputPath
     ): ?string {
         if (empty($slidePaths)) {