feat: Implement end-to-end encryption for channels

- Added EncryptionSalt and WrappedRoomKey properties to Channel model.
- Introduced RoomCrypto class for client-side encryption and decryption.
- Updated ChannelService to handle encrypted channels, including creation and rekeying.
- Modified ChannelsController to expose crypto metadata and rekey functionality.
- Enhanced IrcCommandHandler to block joining encrypted channels over IRC.
- Updated database schema with migration for new encryption fields.
- Refactored file validation and image processing services to accommodate encrypted channels.
- Added unit tests for RoomCrypto functionality and updated existing tests for channel services.

src/EchoHub.Server/Data/EchoHubDbContext.cs

@@ -45,6 +45,8 @@ public class EchoHubDbContext : DbContext
             entity.Property(c => c.Name).IsRequired().HasMaxLength(100);
             entity.Property(c => c.Topic).HasMaxLength(500);
             entity.Property(c => c.PasswordHash).HasMaxLength(100);
+            entity.Property(c => c.EncryptionSalt).HasMaxLength(64);
+            entity.Property(c => c.WrappedRoomKey).HasMaxLength(200);
 
             entity.HasMany(c => c.Messages)
                   .WithOne(m => m.Channel)