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/Infrastructure/Common/INowDateTimeProvider.cs

@@ -0,0 +1,6 @@
+namespace Govor.Application.Infrastructure.Common;
+
+public interface INowDateTimeProvider
+{
+    DateTime Now { get; }
+}

Govor.Application/Infrastructure/Common/NowDateTimeProvider.cs

@@ -0,0 +1,6 @@
+namespace Govor.Application.Infrastructure.Common;
+
+public class NowDateTimeProvider : INowDateTimeProvider
+{
+    public DateTime Now => DateTime.UtcNow;
+}