Batch people lookups when building item DTOs

GetBaseItemDtos already batch fetches user data, child counts, played counts
and artists before its per item loop, but AttachPeople still ran one GetPeople
query per item. Rendering a page of items (for example a large playlist) fired
one extra query per row.

Add GetPeopleByItems to IPeopleRepository, which reads every requested item in a
single query over the people mapping table and returns full PersonInfo (role,
type and sort order) grouped by item id. GetBaseItemDtos prefetches this once
when the People field is requested and passes it into AttachPeople, which reads
from the batch instead of querying per item. The single item GetBaseItemDto path
keeps its existing per item behaviour when no batch is supplied.

Adds a DtoService test asserting people resolve from the batch and the per item
GetPeople is never called.

1 files changed, 50 insertions, 0 deletions

diff --git a/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs b/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
index 6b6240e116..d18f8c6cff 100644
--- a/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
+++ b/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
@@ -14,6 +14,7 @@ using MediaBrowser.Controller.LiveTv;
 using MediaBrowser.Controller.Providers;
 using MediaBrowser.Controller.Trickplay;
 using MediaBrowser.Model.Entities;
+using MediaBrowser.Model.Querying;
 using Moq;
 using Xunit;
 
@@ -155,6 +156,55 @@ public class DtoServiceImageInheritanceTests
         libraryManager.Verify(x => x.GetArtist(It.IsAny<string>(), It.IsAny<DtoOptions>()), Times.Never);
     }
 
+    [Fact]
+    public void GetBaseItemDtos_Items_ResolvePeopleFromBatch_WithoutPerItemLookup()
+    {
+        static MusicAlbum MakeAlbum() => new MusicAlbum
+        {
+            Id = Guid.NewGuid(),
+            Name = "Album",
+            ImageInfos = []
+        };
+
+        var albumOne = MakeAlbum();
+        var albumTwo = MakeAlbum();
+
+        var libraryManager = new Mock<ILibraryManager>();
+
+        // DtoService resolves people for every item in ONE batch (GetPeopleByItems) before the
+        // per-item loop. A regression to the per-item path would call GetPeople(BaseItem) once per
+        // item (the N+1); it is intentionally left unset so such a regression fails here.
+        libraryManager
+            .Setup(x => x.GetPeopleByItems(It.IsAny<IReadOnlyList<Guid>>()))
+            .Returns(new Dictionary<Guid, IReadOnlyList<PersonInfo>>
+            {
+                [albumOne.Id] = [new PersonInfo { ItemId = albumOne.Id, Name = "Some Actor", Type = PersonKind.Actor }],
+                [albumTwo.Id] = [new PersonInfo { ItemId = albumTwo.Id, Name = "Some Actor", Type = PersonKind.Actor }]
+            });
+
+        // AttachPeople still resolves each distinct name to its Person entity to attach images.
+        libraryManager
+            .Setup(x => x.GetPerson("Some Actor"))
+            .Returns(new Person { Id = Guid.NewGuid(), Name = "Some Actor" });
+
+        var dtoService = BuildDtoService(libraryManager);
+
+        var options = new DtoOptions(false) { Fields = [ItemFields.People] };
+        var dtos = dtoService.GetBaseItemDtos([albumOne, albumTwo], options);
+
+        Assert.Equal(2, dtos.Count);
+        foreach (var dto in dtos)
+        {
+            Assert.NotNull(dto.People);
+            Assert.Single(dto.People);
+            Assert.Equal("Some Actor", dto.People[0].Name);
+        }
+
+        // People are batched once for the whole set, never once per item.
+        libraryManager.Verify(x => x.GetPeopleByItems(It.IsAny<IReadOnlyList<Guid>>()), Times.Once);
+        libraryManager.Verify(x => x.GetPeople(It.IsAny<BaseItem>()), Times.Never);
+    }
+
     private static DtoService BuildDtoService(BaseItem displayParent)
     {
         var libraryManager = new Mock<ILibraryManager>();