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>

ProjectSettings/DynamicsManager.asset

@@ -0,0 +1,38 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!55 &1
+PhysicsManager:
+  m_ObjectHideFlags: 0
+  serializedVersion: 13
+  m_Gravity: {x: 0, y: -9.81, z: 0}
+  m_DefaultMaterial: {fileID: 0}
+  m_BounceThreshold: 2
+  m_DefaultMaxDepenetrationVelocity: 10
+  m_SleepThreshold: 0.005
+  m_DefaultContactOffset: 0.01
+  m_DefaultSolverIterations: 6
+  m_DefaultSolverVelocityIterations: 1
+  m_QueriesHitBackfaces: 0
+  m_QueriesHitTriggers: 1
+  m_EnableAdaptiveForce: 0
+  m_ClothInterCollisionDistance: 0
+  m_ClothInterCollisionStiffness: 0
+  m_ContactsGeneration: 1
+  m_LayerCollisionMatrix: c95f10ffc80010ffc80010ffffffffffc80010ffc80010ffffffffffffffffffc94630ffc92510ffc90b10ffc91410ffc94810ffc80218ffc91110ffc80012ffc80010ffc88000ffc80000ffc82010ffffff19ffc80100ffc80080ffc80040ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+  m_AutoSimulation: 1
+  m_AutoSyncTransforms: 1
+  m_ReuseCollisionCallbacks: 0
+  m_ClothInterCollisionSettingsToggle: 0
+  m_ClothGravity: {x: 0, y: -9.81, z: 0}
+  m_ContactPairsMode: 0
+  m_BroadphaseType: 0
+  m_WorldBounds:
+    m_Center: {x: 0, y: 0, z: 0}
+    m_Extent: {x: 250, y: 250, z: 250}
+  m_WorldSubdivisions: 8
+  m_FrictionType: 0
+  m_EnableEnhancedDeterminism: 0
+  m_EnableUnifiedHeightmaps: 1
+  m_ImprovedPatchFriction: 0
+  m_SolverType: 0
+  m_DefaultMaxAngularSpeed: 7