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.Contracts/Responses/SignalR/MessageReadResponse.cs

@@ -0,0 +1,13 @@
+using Govor.Domain.Models.Messages;
+
+namespace Govor.Contracts.Responses.SignalR;
+
+public class MessageReadResponse
+{
+    public required Guid ViewId  { get; set; }
+    public required Guid MessageId { get; set; }
+    public required Guid ReaderId { get; set; }
+    public required Guid RecipientId { get; set; }
+    public required DateTime WhenWas { get; set; }
+    public RecipientType RecipientType { get; set; }
+}