Let users browse followers and following lists

Following lists are public; followers are visible to the user and to mutual followers.
Adds GET /users/{id}/followers and /following, exposes isFollowing and canViewFollowers
on the profile, and returns isFollowedByViewer from user search so the web player no
longer calls /is-following per user.

backend/Tests/Musify.Application.Tests/Users/GetUsersQueryHandlerTests.cs

@@ -1,5 +1,6 @@
 using Musify.Application.Tests.TestSupport;
 using Musify.Application.Users;
+using Musify.Domain.Entities;
 using Xunit;
 
 namespace Musify.Application.Tests.Users
@@ -30,5 +31,18 @@ namespace Musify.Application.Tests.Users
             var user = Assert.Single(result.Value.Items);
             Assert.Equal(1, user.Id);
         }
+
+        [Fact]
+        public async Task Handle_Viewer_MarksFollowedUsers()
+        {
+            await SeedAsync(
+                TestEntities.User(1, "viewer"), TestEntities.User(2, "alice"), TestEntities.User(3, "bob"),
+                new UserFollow { FollowerId = 1, FollowedId = 2 });
+
+            var result = await CreateHandler().Handle(new GetUsersQuery(PageNumber: 1, PageSize: 10, UsernameSearch: null, ViewerId: 1), TestContext.Current.CancellationToken);
+
+            Assert.False(result.IsError);
+            Assert.All(result.Value.Items, item => Assert.Equal(item.Id == 2, item.IsFollowedByViewer));
+        }
     }
 }