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.Core/Contracts/IMessageEncryptionService.cs

@@ -0,0 +1,14 @@
+namespace EchoHub.Core.Contracts;
+
+public interface IMessageEncryptionService
+{
+    /// <summary>
+    /// Whether database content should be encrypted at rest (server setting).
+    /// </summary>
+    bool EncryptDatabaseEnabled { get; }
+
+    string Encrypt(string plaintext);
+    string Decrypt(string content);
+    string? EncryptNullable(string? value);
+    string? DecryptNullable(string? value);
+}

src/EchoHub.Core/DTOs/ServerDtos.cs

@@ -1,3 +1,5 @@
 namespace EchoHub.Core.DTOs;
 
 public record ServerStatusDto(string Name, string? Description, int OnlineUsers, int TotalChannels);
+
+public record EncryptionKeyResponse(string Key);