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

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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);
+    }
+}