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/DataTransferObjects/Albums/RequestAlbumPictureUploadRequest.cs

@@ -1,6 +0,0 @@
-namespace Musify.Api.DataTransferObjects.Albums;
-
-public record RequestAlbumPictureUploadRequest(
-    string FileType,
-    string ContentType,
-    long? ExpectedSizeBytes = null);

backend/Hosts/Musify.Api/DataTransferObjects/Albums/UpdateAlbumRequest.cs

@@ -1,7 +1,7 @@
 namespace Musify.Api.DataTransferObjects.Albums;
 
 public record UpdateAlbumRequest(
-    string NewTitle,
-    string? NewDescription,
-    int? NewReleaseYear,
-    Guid? NewPictureIntentId);
+    string Title,
+    string? Description,
+    int? ReleaseYear,
+    Guid? PictureIntentId);