Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

backend/Core/Musify.Application/Contracts/AudioTranscodeResult.cs

@@ -0,0 +1,4 @@
+namespace Musify.Application.Contracts
+{
+    public record AudioTranscodeResult(int StatusCode, TimeSpan Duration);
+}

backend/Core/Musify.Application/Contracts/IAudioTranscoderService.cs

@@ -0,0 +1,9 @@
+namespace Musify.Application.Contracts
+{
+    public interface IAudioTranscoderService
+    {
+        Task<AudioTranscodeResult> TranscodeToAudioFileAsync(Stream audioStream, string destinationPath, CancellationToken cancellationToken);
+
+        Task<bool> IsValidAudioFileAsync(string filePath, CancellationToken cancellationToken);
+    }
+}

backend/Core/Musify.Application/Contracts/IDatabase.cs

@@ -0,0 +1,48 @@
+using Microsoft.EntityFrameworkCore;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Contracts
+{
+    public interface IDatabase
+    {
+        DbSet<User> Users { get; }
+
+        DbSet<PlayList> PlayLists { get; }
+
+        DbSet<Album> Albums { get; }
+
+        DbSet<UserAlbum> UserAlbums { get; }
+
+        DbSet<ExternalAlbum> ExternalAlbums { get; }
+
+        DbSet<AlbumHasTrack> AlbumHasTracks { get; }
+
+        DbSet<Track> Tracks { get; }
+
+        DbSet<LocalTrack> LocalTracks { get; }
+
+        DbSet<ExternalTrack> ExternalTracks { get; }
+
+        DbSet<Artist> Artists { get; }
+
+        DbSet<TrackArtist> TrackArtists { get; }
+
+        DbSet<UserHasTrack> UserHasTracks { get; }
+
+        DbSet<PlayListHasTrack> PlayListHasTracks { get; }
+
+        DbSet<Mix> Mixes { get; }
+
+        DbSet<MixItem> MixItems { get; }
+
+        DbSet<Upload> Uploads { get; }
+
+        DbSet<UploadIntent> UploadIntents { get; }
+
+        DbSet<ListeningHistory> ListeningHistories { get; }
+
+        Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken);
+
+        Task<int> SaveChangesAsync(CancellationToken cancellationToken);
+    }
+}

backend/Core/Musify.Application/Contracts/IDatabaseTransaction.cs

@@ -0,0 +1,7 @@
+namespace Musify.Application.Contracts
+{
+    public interface IDatabaseTransaction : IAsyncDisposable
+    {
+        Task CommitAsync(CancellationToken cancellationToken = default);
+    }
+}

backend/Core/Musify.Application/Contracts/IEventBus.cs

@@ -0,0 +1,7 @@
+namespace Musify.Application.Contracts
+{
+    public interface IEventBus
+    {
+        Task PublishAsync<T>(T message, CancellationToken cancellationToken) where T : class;
+    }
+}

backend/Core/Musify.Application/Contracts/IPictureService.cs

@@ -0,0 +1,7 @@
+namespace Musify.Application.Contracts
+{
+    public interface IPictureService
+    {
+        Task<Stream> ResizePictureAsWebpAsync(Stream pictureStream, int width, int height, CancellationToken cancellationToken);
+    }
+}

backend/Core/Musify.Application/Contracts/IStorageService.cs

@@ -0,0 +1,28 @@
+namespace Musify.Application.Contracts
+{
+    public interface IStorageService
+    {
+        Task<Stream> GetFileAsync(string bucket, string key, CancellationToken cancellationToken);
+
+        Task<ObjectMetaData?> HeadObjectAsync(string bucket, string key, CancellationToken cancellationToken);
+
+        Task<string> GetUrlAsync(string bucket, string key, TimeSpan ExpirationTime, CancellationToken cancellationToken);
+
+        Task<string> GetUploadUrlAsync(string bucket, string key, string contentType, TimeSpan expirationTime, CancellationToken cancellationToken, bool preventOverwrite = true);
+
+        Task UploadFileAsync(string filePath, string bucket, string key, CancellationToken cancellationToken);
+
+        Task UploadFileAsync(Stream sourceStream, string contentType, string bucket, string key, CancellationToken cancellationToken);
+
+        Task<IReadOnlyList<string>> TransferFilesAsync(string sourceDirectory, string bucket, string route, CancellationToken cancellationToken);
+
+        Task RemoveFileAsync(string bucket, string key, CancellationToken cancellationToken);
+
+        Task CopyFileAsync(string sourceBucket, string sourceKey, string destinationBucket, string destinationKey,
+            CancellationToken cancellationToken);
+
+        Task RemoveFolderAsync(string bucket, string folderKey, CancellationToken cancellationToken);
+        
+        IAsyncEnumerable<StorageObject> ListObjectsAsync(string bucket, string prefix, CancellationToken cancellationToken);
+    }
+}

backend/Core/Musify.Application/Contracts/IStreamTicketService.cs

@@ -0,0 +1,9 @@
+namespace Musify.Application.Contracts
+{
+    public interface IStreamTicketService
+    {
+        StreamTicket IssueTicket(long userId, string keyPrefix);
+    }
+
+    public record StreamTicket(string Token, int ExpiresInSeconds);
+}

backend/Core/Musify.Application/Contracts/IYouTubeDownloaderService.cs

@@ -0,0 +1,9 @@
+namespace Musify.Application.Contracts
+{
+    public record YouTubeDownloadResult(int ExitCode, string StandardError);
+
+    public interface IYouTubeDownloaderService
+    {
+        Task<YouTubeDownloadResult> DownloadAudioAsync(string videoId, string destinationFilePath, CancellationToken cancellationToken);
+    }
+}

backend/Core/Musify.Application/Contracts/IYouTubeMusicService.cs

@@ -0,0 +1,64 @@
+using ErrorOr;
+
+namespace Musify.Application.Contracts
+{
+    public record YouTubeArtistRef(string? Id, string Name);
+
+    public record YouTubeSongResult(
+        string VideoId,
+        string Title,
+        string Artist,
+        string Album,
+        int DurationSeconds,
+        string ThumbnailUrl,
+        bool IsExplicit,
+        IReadOnlyList<YouTubeArtistRef> Artists);
+
+    public record YouTubeSearchResult(
+        IReadOnlyList<YouTubeSongResult> Items,
+        string ContinuationToken);
+
+    public record YouTubeAlbumResult(
+        string AlbumId,
+        string Title,
+        string Artist,
+        string ThumbnailUrl,
+        int? ReleaseYear,
+        bool IsSingle,
+        bool IsEp,
+        IReadOnlyList<YouTubeArtistRef> Artists);
+
+    public record YouTubeAlbumSearchResult(
+        IReadOnlyList<YouTubeAlbumResult> Items,
+        string ContinuationToken);
+
+    public record YouTubeAlbumTrack(
+        string VideoId,
+        string Title,
+        int DurationSeconds,
+        int TrackNumber,
+        bool IsExplicit);
+
+    public record YouTubeAlbumDetail(
+        YouTubeAlbumResult Album,
+        string Description,
+        int TotalDurationSeconds,
+        IReadOnlyList<YouTubeAlbumTrack> Tracks);
+
+    public record YouTubeStreamInfo(string Url, int ExpiresInSeconds);
+
+    public interface IYouTubeMusicService
+    {
+        Task<ErrorOr<YouTubeSearchResult>> SearchSongsAsync(string query, string continuationToken, CancellationToken cancellationToken);
+
+        Task<ErrorOr<YouTubeAlbumSearchResult>> SearchAlbumsAsync(string query, string continuationToken, CancellationToken cancellationToken);
+
+        Task<ErrorOr<YouTubeAlbumDetail>> GetAlbumAsync(string albumId, CancellationToken cancellationToken);
+
+        Task<ErrorOr<YouTubeStreamInfo>> GetAudioStreamAsync(string videoId, CancellationToken cancellationToken);
+
+        Task<ErrorOr<YouTubeSongResult>> GetSongAsync(string videoId, CancellationToken cancellationToken);
+
+        string ResolveArtworkUrl(string thumbnailUrl);
+    }
+}

backend/Core/Musify.Application/Contracts/ObjectMetaData.cs

@@ -0,0 +1,4 @@
+namespace Musify.Application.Contracts
+{
+    public record ObjectMetaData(string ContentType, long ContentLength);
+}

backend/Core/Musify.Application/Contracts/StorageObject.cs

@@ -0,0 +1,4 @@
+namespace Musify.Application.Contracts
+{
+    public record StorageObject(string Key, DateTime LastModifiedUtc);
+}