Fix query and API bugs: 403 for non-owners, page bounds, lifecycle filters, cover fallback, race-safe inserts, async album delete

backend/Core/Musify.Application/Shared/AppErrors.cs

@@ -0,0 +1,15 @@
+using ErrorOr;
+
+namespace Musify.Application.Shared;
+
+public static class AppErrors
+{
+    public static Error NotFound(string entity, object id) =>
+        Error.NotFound($"{entity}.NotFound", $"{entity} {id} was not found.");
+
+    public static Error Forbidden(string entity, object id) =>
+        Error.Forbidden($"{entity}.Forbidden", $"You do not have permission to modify {entity} {id}.");
+
+    public static Error Conflict(string code, string description) =>
+        Error.Conflict(code, description);
+}

backend/Core/Musify.Application/Shared/Pagination.cs

@@ -0,0 +1,6 @@
+namespace Musify.Application.Shared;
+
+public record PageRequest(int PageNumber = 1, int PageSize = 20)
+{
+    public const int MaxPageSize = 100;
+}