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.Application/Services/UploadIntentValidator.cs

@@ -1,4 +1,4 @@
-using Ardalis.Result;
+using ErrorOr;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Configuration;
 using Musify.Domain.Entities;
@@ -11,7 +11,7 @@ public sealed class UploadIntentValidator(
     IDatabase database,
     IStorageService storageService)
 {
-    public async Task<Result> CheckQuotaAsync(
+    public async Task<ErrorOr<Success>> CheckQuotaAsync(
         UploadIntentConfiguration config,
         long userId,
         long requiredBytes,
@@ -24,16 +24,16 @@ public sealed class UploadIntentValidator(
             .ToListAsync(cancellationToken);
 
         if (activeIntents.Count + requiredIntentCount > config.MaxActiveUploadIntentsPerUser)
-            return Result.Invalid(new ValidationError("Upload intent limit exceeded. Wait for existing uploads to complete or expire."));
+            return Error.Validation(description: "Upload intent limit exceeded. Wait for existing uploads to complete or expire.");
 
         var activeBytes = activeIntents.Sum(i => i.ExpectedSizeBytes ?? config.DefaultExpectedPictureSizeBytes);
         if (activeBytes + requiredBytes > config.MaxActiveUploadBytesPerUser)
-            return Result.Invalid(new ValidationError("Upload byte quota exceeded. Wait for existing uploads to complete or expire."));
+            return Error.Validation(description: "Upload byte quota exceeded. Wait for existing uploads to complete or expire.");
 
-        return Result.Success();
+        return new Success();
     }
 
-    public async Task<Result<UploadIntent>> ValidateAndLoadAsync(
+    public async Task<ErrorOr<UploadIntent>> ValidateAndLoadAsync(
         UploadIntentConfiguration config,
         Guid intentId,
         long userId,
@@ -43,21 +43,21 @@ public sealed class UploadIntentValidator(
             .FirstOrDefaultAsync(i => i.Id == intentId, cancellationToken);
 
         if (intent is null || intent.UserId != userId)
-            return Result.NotFound("Upload intent not found or not accessible.");
+            return Error.NotFound(description: "Upload intent not found or not accessible.");
 
         if (intent.Status == UploadIntentStatus.Consumed)
-            return Result.Conflict("Upload intent has already been consumed.");
+            return Error.Conflict(description: "Upload intent has already been consumed.");
 
         if (intent.Status == UploadIntentStatus.Expired || intent.ExpiresAt < DateTime.UtcNow)
-            return Result.Invalid(new ValidationError("Upload intent has expired."));
+            return Error.Validation(description: "Upload intent has expired.");
 
         var metadata = await storageService.HeadObjectAsync(intent.Bucket, intent.Key, cancellationToken);
         if (metadata is null)
-            return Result.NotFound("Uploaded object not found in storage. Upload the file first.");
+            return Error.NotFound(description: "Uploaded object not found in storage. Upload the file first.");
 
         if (metadata.ContentLength > config.MaxUploadBytes)
-            return Result.Invalid(new ValidationError($"Uploaded file exceeds the maximum allowed size of {config.MaxUploadBytes} bytes."));
+            return Error.Validation(description: $"Uploaded file exceeds the maximum allowed size of {config.MaxUploadBytes} bytes.");
 
-        return Result.Success(intent);
+        return intent;
     }
 }