feat: enhance message decryption for IRC clients and improve attachment handling

src/EchoHub.Core/Contracts/IMessageEncryptionService.cs

@@ -2,6 +2,12 @@ namespace EchoHub.Core.Contracts;
 
 public interface IMessageEncryptionService
 {
+    /// <summary>
+    /// Prefix marking transport/at-rest encrypted content. <see cref="Decrypt"/> is a
+    /// pass-through for values without it.
+    /// </summary>
+    const string CiphertextPrefix = "$ENC$v1$";
+
     /// <summary>
     /// Whether database content should be encrypted at rest (server setting).
     /// </summary>