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);
+    }
+}

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

@@ -1,25 +0,0 @@
-using X.PagedList;
-
-namespace Musify.Application.Shared;
-
-public record PaginatedResponse<T>(
-    IReadOnlyCollection<T> Items,
-    int PageNumber,
-    int PageSize,
-    int PageCount,
-    int TotalItemCount,
-    bool HasNextPage,
-    bool HasPreviousPage)
-{
-    public static PaginatedResponse<T> FromPagedList(IPagedList<T> pagedList)
-    {
-        return new PaginatedResponse<T>(
-            pagedList.ToArray(),
-            pagedList.PageNumber,
-            pagedList.PageSize,
-            pagedList.PageCount,
-            pagedList.TotalItemCount,
-            pagedList.HasNextPage,
-            pagedList.HasPreviousPage);
-    }
-}

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

@@ -1,6 +1,45 @@
+using Microsoft.EntityFrameworkCore;
+
 namespace Musify.Application.Shared;
 
 public record PageRequest(int PageNumber = 1, int PageSize = 20)
 {
     public const int MaxPageSize = 100;
 }
+
+public record PaginatedResponse<T>(
+    IReadOnlyList<T> Items,
+    int PageNumber,
+    int PageSize,
+    int PageCount,
+    int TotalItemCount,
+    bool HasNextPage,
+    bool HasPreviousPage);
+
+public static class PaginationExtensions
+{
+    /// <summary>Counts and pages the same (already filtered, ordered and projected) query.</summary>
+    public static async Task<PaginatedResponse<T>> ToPaginatedAsync<T>(
+        this IQueryable<T> query, PageRequest page, CancellationToken cancellationToken)
+    {
+        var pageNumber = Math.Max(page.PageNumber, 1);
+        var pageSize = Math.Clamp(page.PageSize, 1, PageRequest.MaxPageSize);
+
+        var totalCount = await query.CountAsync(cancellationToken);
+        var items = await query
+            .Skip((pageNumber - 1) * pageSize)
+            .Take(pageSize)
+            .ToListAsync(cancellationToken);
+
+        var pageCount = (int)Math.Ceiling(totalCount / (double)pageSize);
+
+        return new PaginatedResponse<T>(
+            items,
+            pageNumber,
+            pageSize,
+            pageCount,
+            totalCount,
+            HasNextPage: pageNumber < pageCount,
+            HasPreviousPage: pageNumber > 1 && pageCount > 0);
+    }
+}

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

@@ -0,0 +1,6 @@
+namespace Musify.Application.Shared;
+
+public static class TextNormalizer
+{
+    public static string Normalize(string value) => value.Trim().ToUpperInvariant();
+}