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/Controllers/ServerController.cs

@@ -1,6 +1,8 @@
 using EchoHub.Core.DTOs;
 using EchoHub.Server.Data;
+using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.RateLimiting;
 using Microsoft.EntityFrameworkCore;
 
 namespace EchoHub.Server.Controllers;
@@ -32,4 +34,17 @@ public class ServerController : ControllerBase
 
         return Ok(status);
     }
+
+    [HttpGet("encryption-key")]
+    [Authorize]
+    [EnableRateLimiting("auth")]
+    public IActionResult GetEncryptionKey()
+    {
+        var key = _config["Encryption:Key"];
+
+        if (string.IsNullOrEmpty(key))
+            return StatusCode(503, new ErrorResponse("Encryption is not configured on this server."));
+
+        return Ok(new EncryptionKeyResponse(key));
+    }
 }