feat: add MergePeople to person repository

Reassigns albums (PersonOwnerId), faces (PersonId), and maintainers
from source people to destination, then hard-deletes source people.

Lactose/Repositories/IPersonRepository.cs

@@ -89,6 +89,14 @@ public interface IPersonRepository : IDisposable {
     /// <returns>A collection of matching people.</returns>
     IEnumerable<Person> FindBulk(IEnumerable<Guid> ids);
 
+    /// <summary>
+    /// Merges multiple source people into a destination person.
+    /// Albums, faces, and maintainers are reassigned to the destination, then source people are hard-deleted.
+    /// </summary>
+    /// <param name="destId">The ID of the destination person.</param>
+    /// <param name="sourceIds">The IDs of the source people to merge.</param>
+    void MergePeople(Guid destId, List<Guid> sourceIds);
+
     /// <summary>
     /// Removes a person from the database.
     /// </summary>

Lactose/Repositories/PersonRepository.cs

@@ -227,6 +227,39 @@ public class PersonRepository(LactoseDbContext context) : IPersonRepository {
         }
     }
 
+    /// <inheritdoc />
+    public void MergePeople(Guid destId, List<Guid> sourceIds) {
+        var dest = context.People
+            .AsSplitQuery()
+            .Include(p => p.Maintainers)
+            .FirstOrDefault(p => p.Id == destId);
+        if (dest == null) return;
+
+        // Reassign albums
+        var albums = context.Albums.Where(a => sourceIds.Contains(a.PersonOwnerId ?? Guid.Empty)).ToList();
+        foreach (var album in albums)
+            album.PersonOwnerId = destId;
+
+        // Reassign faces
+        var faces = context.Faces.Where(f => f.PersonId.HasValue && sourceIds.Contains(f.PersonId.Value)).ToList();
+        foreach (var face in faces)
+            face.PersonId = destId;
+
+        // Reassign maintainers
+        var existingMaintainerIds = dest.Maintainers?.Select(m => m.Id).ToHashSet() ?? [];
+        var sourceMaintainers = context.PersonMaintainers
+            .Where(pm => sourceIds.Contains(pm.PersonId))
+            .ToList();
+        foreach (var pm in sourceMaintainers) {
+            if (!existingMaintainerIds.Contains(pm.UserId))
+                context.PersonMaintainers.Add(new PersonMaintainer { PersonId = destId, UserId = pm.UserId });
+        }
+
+        // Hard-delete source people
+        var people = context.People.Where(p => sourceIds.Contains(p.Id)).ToList();
+        context.People.RemoveRange(people);
+    }
+
     /// <inheritdoc />
     public void Dispose() => context.Dispose();
 }