feat: launcher.exe와 game.zip 별도 업로드/서빙 분리

- POST /api/download/upload/game - 게임 zip 업로드
- POST /api/download/upload/launcher - launcher.exe 업로드
- GET /api/download/launcher - launcher.exe 서빙
- Info 모델에 LauncherURL, LauncherSize 필드 추가
- Content-Disposition 헤더로 올바른 파일명 설정

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

routes/routes.go

@@ -39,5 +39,7 @@ func Register(
 	dl := api.Group("/download")
 	dl.Get("/info", dlH.GetInfo)
 	dl.Get("/file", dlH.ServeFile)
-	dl.Post("/upload", middleware.Auth, middleware.AdminOnly, dlH.Upload)
+	dl.Get("/launcher", dlH.ServeLauncher)
+	dl.Post("/upload/game", middleware.Auth, middleware.AdminOnly, dlH.Upload)
+	dl.Post("/upload/launcher", middleware.Auth, middleware.AdminOnly, dlH.UploadLauncher)
 }