Add read receipts and editing limits

Introduced message read tracking with a dedicated MessageReadingService, SignalR read request/response contracts, and hub notification flow for read receipts. Added configurable message edit constraints through MessageEditingOptions and a shared INowDateTimeProvider so timestamps are consistent across the app. Also fixed recipient validation in message removal and improved push token upsert behavior by matching the correct user/session record.

Govor.Application/Messages/IMessageReadingService.cs

@@ -0,0 +1,10 @@
+using Govor.Domain.Common;
+using Govor.Domain.Models.Messages;
+using SmartRes;
+
+namespace Govor.Application.Messages;
+
+public interface IMessageReadingService
+{
+    Task<Result<Message, Error>> ReadMessageAsync(Guid readerId, Guid messageId);
+}