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/GetUserProfileQueryHandlerTests.cs

@@ -30,6 +30,46 @@ namespace Musify.Application.Tests.Users
             Assert.Equal(1, result.Value.FollowingCount);
         }
 
+        [Fact]
+        public async Task Handle_MutualFollow_ViewerCanSeeFollowersAndIsFollowing()
+        {
+            var viewer = TestEntities.User(1, "viewer");
+            var user = TestEntities.User(2, "user");
+            await SeedAsync(
+                viewer, user,
+                new UserFollow { FollowerId = viewer.Id, FollowedId = user.Id },
+                new UserFollow { FollowerId = user.Id, FollowedId = viewer.Id });
+
+            var result = await CreateHandler().Handle(new GetUserProfileQuery(user.Id, viewer.Id), TestContext.Current.CancellationToken);
+
+            Assert.True(result.Value.IsFollowing);
+            Assert.True(result.Value.CanViewFollowers);
+        }
+
+        [Fact]
+        public async Task Handle_OneWayFollow_ViewerCannotSeeFollowers()
+        {
+            var viewer = TestEntities.User(1, "viewer");
+            var user = TestEntities.User(2, "user");
+            await SeedAsync(viewer, user, new UserFollow { FollowerId = viewer.Id, FollowedId = user.Id });
+
+            var result = await CreateHandler().Handle(new GetUserProfileQuery(user.Id, viewer.Id), TestContext.Current.CancellationToken);
+
+            Assert.True(result.Value.IsFollowing);
+            Assert.False(result.Value.CanViewFollowers);
+        }
+
+        [Fact]
+        public async Task Handle_OwnProfile_CanSeeFollowersButIsNotFollowing()
+        {
+            await SeedAsync(TestEntities.User(1, "user"));
+
+            var result = await CreateHandler().Handle(new GetUserProfileQuery(1, 1), TestContext.Current.CancellationToken);
+
+            Assert.False(result.Value.IsFollowing);
+            Assert.True(result.Value.CanViewFollowers);
+        }
+
         [Fact]
         public async Task Handle_UserMissing_ReturnsNotFound()
         {