refactor: replace Ardalis.Result with ErrorOr across the app

Ardalis Result<T> carried incoherent success sub-statuses and a clunky
generic converter; ErrorOr models success-or-errors cleanly.

- Application: all commands/queries/handlers now return ErrorOr<T>
  (non-generic Result -> ErrorOr<Success>); errors via Error.NotFound/
  Conflict/Validation/Unauthorized/Failure; success via value or new Success()
- Drop ResultExtensions.As<>(): error propagation is now just return .Errors
- Rewrite HTTP mapping (ErrorOrHttpExtensions) off Error.Type; success maps
  to 200 (value) or 204 (Success); Created still built in the endpoints
- Endpoints/gateway middleware: !IsSuccess -> IsError
- Swap Ardalis.Result package for ErrorOr 2.1.1 (remove unused ref from Worker)

Build: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.StreamingGateway/Authentication/TicketValidator.cs

@@ -1,5 +1,5 @@
 using System.Security.Cryptography;
-using Ardalis.Result;
+using ErrorOr;
 using Microsoft.Extensions.Options;
 using Microsoft.IdentityModel.JsonWebTokens;
 using Microsoft.IdentityModel.Tokens;
@@ -35,25 +35,25 @@ public sealed class TicketValidator : IDisposable
         };
     }
 
-    public async Task<Result<string>> ValidateTicketAsync(string? token)
+    public async Task<ErrorOr<string>> ValidateTicketAsync(string? token)
     {
         if (string.IsNullOrWhiteSpace(token))
-            return Result.Unauthorized();
+            return Error.Unauthorized();
 
         var result = await _handler.ValidateTokenAsync(token, _validationParameters);
         if (!result.IsValid)
-            return Result.Unauthorized();
+            return Error.Unauthorized();
 
         if (!result.Claims.TryGetValue("prefix", out var claim))
-            return Result.Unauthorized();
+            return Error.Unauthorized();
 
         if (claim is not string prefix)
-            return Result.Unauthorized();
+            return Error.Unauthorized();
 
         if (string.IsNullOrEmpty(prefix))
-            return Result.Unauthorized();
+            return Error.Unauthorized();
 
-        return Result.Success(prefix);
+        return prefix;
     }
 
     public void Dispose() => _rsa.Dispose();