chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

backend/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);
+    }
+}