feat: Add invite codes and message replies functionality

- Introduced a new migration to add InviteCodes table and ReplyToMessageId column in Messages.
- Updated ChatHub to support replying to messages.
- Enhanced ChatService to handle message replies and validate reply targets.
- Modified UserService to implement invite-only registration mode with invite code consumption.
- Added configuration options for registration modes in appsettings.
- Created unit tests for new features including invite code registration and message reply formatting.

src/EchoHub.Server/appsettings.example.json

@@ -14,7 +14,8 @@
     "PublicServer": false,
     "PublicHosts": [],
     "Tags": [],
-    "Admins": []
+    "Admins": [],
+    "Registration": "open"
   },
   "Storage": {
     "CleanupIntervalHours": 1,