feat: implement cross-device reading progress synchronization using SignalR and remove legacy quiz generation services.

src/NexusReader.Infrastructure/Handlers/UpdateReadingProgressCommandHandler.cs

@@ -0,0 +1,46 @@
+using FluentResults;
+using MediatR;
+using Microsoft.AspNetCore.SignalR;
+using Microsoft.EntityFrameworkCore;
+using NexusReader.Application.Commands.Sync;
+using NexusReader.Domain.Entities;
+using NexusReader.Infrastructure.Persistence;
+using NexusReader.Infrastructure.RealTime;
+
+namespace NexusReader.Infrastructure.Handlers;
+
+public class UpdateReadingProgressCommandHandler : IRequestHandler<UpdateReadingProgressCommand, Result>
+{
+    private readonly AppDbContext _context;
+    private readonly IHubContext<SyncHub> _hubContext;
+
+    public UpdateReadingProgressCommandHandler(
+        AppDbContext context, 
+        IHubContext<SyncHub> hubContext)
+    {
+        _context = context;
+        _hubContext = hubContext;
+    }
+
+    public async Task<Result> Handle(UpdateReadingProgressCommand request, CancellationToken cancellationToken)
+    {
+        var user = await _context.Users.FirstOrDefaultAsync(u => u.Id == request.UserId, cancellationToken);
+        if (user == null)
+        {
+            return Result.Fail("User not found.");
+        }
+
+        var now = DateTime.UtcNow;
+        user.LastReadPageId = request.PageId;
+        user.LastReadAt = now;
+
+        await _context.SaveChangesAsync(cancellationToken);
+
+        // Broadcast to other devices
+        await _hubContext.Clients
+            .Group($"User_{request.UserId}")
+            .SendAsync("ProgressUpdated", request.PageId, now, cancellationToken);
+
+        return Result.Ok();
+    }
+}