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/Albums/CreateAlbumRequestValidator.cs

@@ -9,13 +9,13 @@ public sealed class CreateAlbumRequestValidator : AbstractValidator<CreateAlbumR
     {
         RuleFor(x => x.Title)
             .NotEmpty()
-            .MaximumLength(200);
+            .MaximumLength(Limits.AlbumTitle);
 
         RuleFor(x => x.Description)
-            .MaximumLength(256);
+            .MaximumLength(Limits.Description);
 
         RuleFor(x => x.ReleaseYear)
-            .InclusiveBetween(AlbumReleaseYear.Earliest, AlbumReleaseYear.Latest)
+            .InclusiveBetween(Limits.EarliestReleaseYear, Limits.LatestReleaseYear)
             .When(x => x.ReleaseYear.HasValue);
 
         RuleFor(x => x.PictureIntentId)