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.

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

backend/Musify.Infrastructure/Persistence/Database.cs

@@ -0,0 +1,57 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Storage;
+using MassTransit;
+using Musify.Domain.Entities;
+using Musify.Infrastructure.Configuration;
+using AppIDatabase = Musify.Application.Contracts.IDatabase;
+using Musify.Application.Contracts;
+
+namespace Musify.Infrastructure.Persistence
+{
+    public class Database(DatabaseConfiguration configuration)
+        : DbContext, AppIDatabase
+    {
+        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+        {
+            optionsBuilder.UseNpgsql(configuration.ConnectionString);
+            optionsBuilder.AddInterceptors(new AuditableEntityInterceptor());
+        }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            base.OnModelCreating(modelBuilder);
+
+            modelBuilder.AddInboxStateEntity();
+            modelBuilder.AddOutboxMessageEntity();
+            modelBuilder.AddOutboxStateEntity();
+        }
+
+        public DbSet<User> Users => Set<User>();
+
+        public DbSet<Track> Tracks => Set<Track>();
+
+        public DbSet<PlayList> PlayLists => Set<PlayList>();
+
+        public DbSet<UserHasTrack> UserHasTracks => Set<UserHasTrack>();
+
+        public DbSet<PlayListHasTrack> PlayListHasTracks => Set<PlayListHasTrack>();
+
+        public DbSet<Upload> Uploads => Set<Upload>();
+
+        public DbSet<UploadIntent> UploadIntents => Set<UploadIntent>();
+
+        public async Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken)
+        {
+            var transaction = await Database.BeginTransactionAsync(isolationLevel, cancellationToken);
+            return new DatabaseTransaction(transaction);
+        }
+
+        private sealed class DatabaseTransaction(IDbContextTransaction inner) : IDatabaseTransaction
+        {
+            public Task CommitAsync(CancellationToken cancellationToken = default) =>
+                inner.CommitAsync(cancellationToken);
+
+            public ValueTask DisposeAsync() => inner.DisposeAsync();
+        }
+    }
+}