refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Application/Contracts/IAudioTranscoderService.cs

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

Musify.Application/Contracts/IDatabase.cs

@@ -0,0 +1,26 @@
+using Microsoft.EntityFrameworkCore;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Contracts
+{
+    public interface IDatabase
+    {
+        DbSet<User> Users { get; }
+
+        DbSet<PlayList> PlayLists { get; }
+
+        DbSet<Track> Tracks { get; }
+
+        DbSet<UserHasTrack> UserHasTracks { get; }
+
+        DbSet<PlayListHasTrack> PlayListHasTracks { get; }
+
+        DbSet<Upload> Uploads { get; }
+
+        DbSet<UploadIntent> UploadIntents { get; }
+
+        Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken);
+
+        Task<int> SaveChangesAsync(CancellationToken cancellationToken);
+    }
+}

Musify.Application/Contracts/IDatabaseTransaction.cs

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

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;
+    }
+}

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);
+    }
+}

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 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);
+    }
+}

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);
+}

Musify.Application/Contracts/ObjectMetaData.cs

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

Musify.Application/Contracts/StorageObject.cs

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