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/Users/GetUserById.cs

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+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Ping.Application.Contracts;
+using Ping.Application.Users.Responses;
+
+namespace Ping.Application.Users
+{
+    public record GetUserByIdQuery(long Id) : IQuery<ErrorOr<UserResponse>>;
+
+    public class GetUserByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserByIdQuery, ErrorOr<UserResponse>>
+    {
+        public async ValueTask<ErrorOr<UserResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users.AsNoTracking().FirstOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
+            return user is null
+                ? Error.NotFound(description: $"User {request.Id} not found")
+                : UserResponse.FromEntity(user);
+        }
+    }
+}

backend/Core/Ping.Application/Users/Responses/UserResponse.cs

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+using System.Text.Json.Serialization;
+using Ping.Application.Serialization;
+using Ping.Domain.Entities;
+
+namespace Ping.Application.Users.Responses
+{
+    public record UserResponse(
+        [property: JsonConverter(typeof(LongAsStringConverter))] long Id,
+        string Username,
+        string? DisplayName,
+        string? AvatarUrl)
+    {
+        public static UserResponse FromEntity(User user) =>
+            new(user.Id, user.Username, user.DisplayName, user.AvatarUrl);
+    }
+}

backend/Core/Ping.Application/Users/SearchUsers.cs

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+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Ping.Application.Contracts;
+using Ping.Application.Users.Responses;
+
+namespace Ping.Application.Users
+{
+    public record SearchUsersQuery(string UsernameQuery, long ExcludingUserId) : IQuery<IReadOnlyList<UserResponse>>;
+
+    public class SearchUsersQueryHandler(IDatabase database)
+        : IQueryHandler<SearchUsersQuery, IReadOnlyList<UserResponse>>
+    {
+        public async ValueTask<IReadOnlyList<UserResponse>> Handle(SearchUsersQuery request, CancellationToken cancellationToken)
+        {
+            var normalizedQuery = request.UsernameQuery.Trim().ToUpperInvariant();
+            if (normalizedQuery.Length == 0)
+                return [];
+
+            var users = await database.Users.AsNoTracking()
+                .Where(u => u.Id != request.ExcludingUserId && u.NormalizedUsername.Contains(normalizedQuery))
+                .OrderBy(u => u.Username)
+                .Take(20)
+                .ToListAsync(cancellationToken);
+
+            return users.Select(UserResponse.FromEntity).ToList();
+        }
+    }
+}

backend/Core/Ping.Application/Users/SyncUser.cs

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+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Ping.Application.Contracts;
+using Ping.Application.Users.Responses;
+using Ping.Domain.Entities;
+
+namespace Ping.Application.Users
+{
+    public record SyncUserCommand(long Id, string Username, string? DisplayName, string? AvatarUrl)
+        : ICommand<UserResponse>;
+
+    public class SyncUserCommandHandler(IDatabase database, ILogger<SyncUserCommandHandler> logger)
+        : ICommandHandler<SyncUserCommand, UserResponse>
+    {
+        public async ValueTask<UserResponse> Handle(SyncUserCommand request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users.FirstOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
+
+            if (user is null)
+            {
+                user = new User
+                {
+                    Id = request.Id,
+                    Username = request.Username,
+                    NormalizedUsername = request.Username.ToUpperInvariant(),
+                    DisplayName = request.DisplayName,
+                    AvatarUrl = request.AvatarUrl,
+                };
+                await database.Users.AddAsync(user, cancellationToken);
+                logger.LogInformation("Provisioned user {UserId} from identity provider", user.Id);
+            }
+            else
+            {
+                user.Username = request.Username;
+                user.NormalizedUsername = request.Username.ToUpperInvariant();
+                user.DisplayName = request.DisplayName;
+                user.AvatarUrl = request.AvatarUrl;
+            }
+
+            await database.SaveChangesAsync(cancellationToken);
+            return UserResponse.FromEntity(user);
+        }
+    }
+}