feat: SongCreator 씬 완성 — Beat Sage URL 지원, info.dat 메타데이터 자동 추출

- BeatSageUploader: audio_url 지원(UploadFromUrl), PollAndDownload 공통화, ZIP 500 오류 3회 재시도
- BeatSageConverter: info.dat 파싱(SongMetadata), BPM 자동 감지 → 노트 타이밍 변환에 적용
- SongCreatorManager: title/BPM 필수 입력 제거, 난이도 4개 자동 선택, GenerateFlowFromUrl 버그 수정
- NasPublisher: audioPath null 허용(URL 흐름에서 로컬 파일 없는 경우 스킵)
- .gitignore/.gitattributes 초기 설정

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

Assets/Readme.asset

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+  title: URP Empty Template
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+  - heading: Welcome to the Universal Render Pipeline
+    text: This template includes the settings and assets you need to start creating with the Universal Render Pipeline.
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+    url:
+  - heading: URP Documentation
+    text:
+    linkText: Read more about URP
+    url: https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@latest
+  - heading: Forums
+    text:
+    linkText: Get answers and support
+    url: https://forum.unity.com/forums/universal-render-pipeline.383/
+  - heading: Report bugs
+    text:
+    linkText: Submit a report
+    url: https://unity3d.com/unity/qa/bug-reporting
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