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.Core/Models/RefreshToken.cs

@@ -0,0 +1,17 @@
+namespace EchoHub.Core.Models;
+
+public class RefreshToken
+{
+    public Guid Id { get; set; }
+    public required string TokenHash { get; set; }
+    public Guid UserId { get; set; }
+    public DateTimeOffset ExpiresAt { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset? RevokedAt { get; set; }
+
+    public bool IsExpired => DateTimeOffset.UtcNow >= ExpiresAt;
+    public bool IsRevoked => RevokedAt is not null;
+    public bool IsActive => !IsExpired && !IsRevoked;
+
+    public User? User { get; set; }
+}