Add Ping backend

.NET 10 solution for Ping, a Discord style chat app: Domain, Application
and Infrastructure projects plus the Api host. SignalR for chat and
presence, LiveKit for call tokens, EF Core on Postgres, S3 compatible
storage for attachments, and OIDC auth shared with the Musify Zitadel
instance. Includes the Compose stack for local development.

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

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore;
+using Ping.Domain.Entities;
+
+namespace Ping.Application.Contracts
+{
+    public interface IDatabase
+    {
+        DbSet<User> Users { get; }
+
+        DbSet<Friendship> Friendships { get; }
+
+        DbSet<Server> Servers { get; }
+
+        DbSet<ServerMember> ServerMembers { get; }
+
+        DbSet<Channel> Channels { get; }
+
+        DbSet<ChannelMember> ChannelMembers { get; }
+
+        DbSet<Message> Messages { get; }
+
+        DbSet<Attachment> Attachments { get; }
+
+        Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken);
+
+        Task<int> SaveChangesAsync(CancellationToken cancellationToken);
+    }
+}