feat: Implement message encryption and decryption support

- Added IMessageEncryptionService and its implementation MessageEncryptionService for handling message encryption.
- Updated ChannelsController and ChatService to encrypt messages before storing and sending.
- Introduced encryption key retrieval endpoint in ServerController.
- Modified EchoHubDbContext to accommodate increased message content and embed JSON lengths for encrypted data.
- Created migrations to support encryption-related database changes.
- Enhanced FirstRunSetup to ensure encryption key is generated if not present.
- Updated appsettings.example.json to include encryption configuration.
- Added comprehensive unit tests for encryption service and compatibility tests between client and server encryption.

src/EchoHub.Server/Program.cs

@@ -109,6 +109,9 @@ while (true)
         builder.Services.AddSingleton<LinkEmbedService>();
         builder.Services.AddHostedService<ServerDirectoryService>();
 
+        // ── Encryption ─────────────────────────────────────────────────────
+        builder.Services.AddSingleton<IMessageEncryptionService, MessageEncryptionService>();
+
         // ── Chat Service + Broadcasters ─────────────────────────────────────
         builder.Services.AddSingleton<IChatBroadcaster, SignalRBroadcaster>();
         builder.Services.AddSingleton<IChatService, ChatService>();