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/Data/Migrations/EchoHubDbContextModelSnapshot.cs

@@ -88,11 +88,11 @@ namespace EchoHub.Server.Data.Migrations
 
                     b.Property<string>("Content")
                         .IsRequired()
-                        .HasMaxLength(2000)
+                        .HasMaxLength(16000)
                         .HasColumnType("TEXT");
 
                     b.Property<string>("EmbedJson")
-                        .HasMaxLength(8000)
+                        .HasMaxLength(32000)
                         .HasColumnType("TEXT");
 
                     b.Property<Guid>("SenderUserId")