Add shared pagination, projections and ownership helpers; simplify query and command handlers

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

@@ -0,0 +1,28 @@
+using ErrorOr;
+using Microsoft.EntityFrameworkCore;
+using Musify.Domain.Abstractions;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Shared;
+
+public static class OwnershipExtensions
+{
+    /// <summary>
+    /// Loads an active, tracked entity owned by <paramref name="userId"/>: NotFound when it is missing
+    /// or not active, Forbidden when someone else owns it.
+    /// </summary>
+    public static async Task<ErrorOr<T>> FindOwnedAsync<T>(
+        this DbSet<T> set, Guid id, long userId, CancellationToken cancellationToken)
+        where T : class, IOwnedEntity, IHasLifeCycle
+    {
+        var entity = await set.SingleOrDefaultAsync(
+            e => e.Id == id && e.LifeCycleStatus == LifeCycleStatus.Active, cancellationToken);
+
+        if (entity == null)
+            return AppErrors.NotFound(typeof(T).Name, id);
+
+        return entity.OwnerUserId == userId
+            ? entity
+            : AppErrors.Forbidden(typeof(T).Name, id);
+    }
+}