Unify API route parameters, return IResult everywhere, merge picture upload DTOs and centralize validation limits

Guid and long route constraints, id/trackId parameter names, Update*Request without the New prefix,
sentence-case operation summaries, shared security scheme id and a single Limits class.

backend/Hosts/Musify.Api/Validators/PictureUploadRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects;
+using Musify.Application.Configuration;
+
+namespace Musify.Api.Validators;
+
+public sealed class PictureUploadRequestValidator : AbstractValidator<PictureUploadRequest>
+{
+    public PictureUploadRequestValidator(UploadIntentConfiguration uploadConfiguration)
+    {
+        RuleFor(x => x.FileType)
+            .MustBeFileType(Uploads.PictureFileTypes);
+
+        RuleFor(x => x.ContentType)
+            .MustBeContentType(Uploads.PictureContentTypes);
+
+        RuleFor(x => x.ExpectedSizeBytes)
+            .MustBeValidUploadSize(uploadConfiguration.MaxUploadBytes);
+    }
+}