feat: Refactor authentication and token management

- Updated EchoHubConnection to use ApiClient for token retrieval.
- Introduced ValidationConstants for common validation patterns and limits.
- Enhanced AuthDtos with refresh token support and expiration details.
- Added new ChatDtos for channel creation and topic updates.
- Created CommonDtos for error and paginated responses.
- Implemented RefreshToken model for managing refresh tokens.
- Modified JwtTokenService to generate and hash refresh tokens.
- Updated AuthController to handle registration, login, token refresh, and logout with improved error handling.
- Enhanced ChannelsController with channel creation, topic updates, and pagination for channel retrieval.
- Added FilesController for file management with rate limiting.
- Improved UsersController for profile updates and avatar uploads with validation.
- Integrated rate limiting across controllers to manage request load.
- Introduced FileValidationHelper for validating uploaded image files.
- Updated database context to include RefreshToken and enforce unique constraints.
- Enhanced ChatHub for improved channel and message handling with validation.
- Updated Program.cs to configure rate limiting and CORS policies.

src/EchoHub.Server/Services/FileValidationHelper.cs

@@ -0,0 +1,68 @@
+namespace EchoHub.Server.Services;
+
+public static class FileValidationHelper
+{
+    private static readonly byte[] JpegMagic = [0xFF, 0xD8, 0xFF];
+    private static readonly byte[] PngMagic = [0x89, 0x50, 0x4E, 0x47];
+    private static readonly byte[] GifMagic = [0x47, 0x49, 0x46];
+    private static readonly byte[] WebpRiff = [0x52, 0x49, 0x46, 0x46]; // "RIFF"
+    private static readonly byte[] WebpTag = [0x57, 0x45, 0x42, 0x50]; // "WEBP"
+
+    /// <summary>
+    /// Validates that a stream contains a recognized image format by checking magic bytes.
+    /// The stream position is reset to the beginning after validation.
+    /// </summary>
+    public static bool IsValidImage(Stream stream)
+    {
+        if (!stream.CanSeek)
+            return false;
+
+        var originalPosition = stream.Position;
+        try
+        {
+            var header = new byte[12];
+            var bytesRead = stream.Read(header, 0, header.Length);
+
+            if (bytesRead < 3)
+                return false;
+
+            // JPEG: FF D8 FF
+            if (StartsWith(header, bytesRead, JpegMagic))
+                return true;
+
+            // PNG: 89 50 4E 47
+            if (bytesRead >= 4 && StartsWith(header, bytesRead, PngMagic))
+                return true;
+
+            // GIF: 47 49 46 (GIF87a or GIF89a)
+            if (StartsWith(header, bytesRead, GifMagic))
+                return true;
+
+            // WebP: RIFF....WEBP
+            if (bytesRead >= 12 && StartsWith(header, bytesRead, WebpRiff)
+                && header[8] == WebpTag[0] && header[9] == WebpTag[1]
+                && header[10] == WebpTag[2] && header[11] == WebpTag[3])
+                return true;
+
+            return false;
+        }
+        finally
+        {
+            stream.Position = originalPosition;
+        }
+    }
+
+    private static bool StartsWith(byte[] buffer, int length, byte[] magic)
+    {
+        if (length < magic.Length)
+            return false;
+
+        for (int i = 0; i < magic.Length; i++)
+        {
+            if (buffer[i] != magic[i])
+                return false;
+        }
+
+        return true;
+    }
+}