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/Data/EchoHubDbContext.cs

@@ -9,6 +9,7 @@ public class EchoHubDbContext(DbContextOptions<EchoHubDbContext> options) : DbCo
     public DbSet<User> Users => Set<User>();
     public DbSet<Channel> Channels => Set<Channel>();
     public DbSet<Message> Messages => Set<Message>();
+    public DbSet<RefreshToken> RefreshTokens => Set<RefreshToken>();
 
     protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
     {
@@ -36,7 +37,7 @@ public class EchoHubDbContext(DbContextOptions<EchoHubDbContext> options) : DbCo
         modelBuilder.Entity<Channel>(entity =>
         {
             entity.HasKey(c => c.Id);
-            entity.HasIndex(c => c.Name);
+            entity.HasIndex(c => c.Name).IsUnique();
             entity.Property(c => c.Name).IsRequired().HasMaxLength(100);
             entity.Property(c => c.Topic).HasMaxLength(500);
 
@@ -56,6 +57,19 @@ public class EchoHubDbContext(DbContextOptions<EchoHubDbContext> options) : DbCo
             entity.Property(m => m.AttachmentFileName).HasMaxLength(255);
         });
 
+        modelBuilder.Entity<RefreshToken>(entity =>
+        {
+            entity.HasKey(r => r.Id);
+            entity.HasIndex(r => r.TokenHash);
+            entity.HasIndex(r => r.UserId);
+            entity.Property(r => r.TokenHash).IsRequired().HasMaxLength(128);
+
+            entity.HasOne(r => r.User)
+                  .WithMany()
+                  .HasForeignKey(r => r.UserId)
+                  .OnDelete(DeleteBehavior.Cascade);
+        });
+
         // SQLite does not support DateTimeOffset in ORDER BY clauses.
         // Convert all DateTimeOffset properties to Unix milliseconds (long) for storage.
         var dateTimeOffsetConverter = new ValueConverter<DateTimeOffset, long>(