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.API/appsettings.json

@@ -19,5 +19,9 @@
   },
   "EncryptionOption": {
     "Secret": "8B2j9kkw9xP5m7nQwE2zY3A-=Q8zP7C4+TqLZpg" 
+  },
+  "MessageEditingOptions": {
+    "Enabled": true,
+    "MaxEditTimeMinutes": 15
   }
 }