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/Core/Musify.Application/Users/FollowVisibility.cs

@@ -0,0 +1,30 @@
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Users
+{
+    public static class FollowVisibility
+    {
+        public static async Task<bool> CanViewFollowersAsync(
+            IDatabase database,
+            long? viewerId,
+            long userId,
+            CancellationToken cancellationToken)
+        {
+            if (viewerId is not { } viewer)
+                return false;
+
+            if (viewer == userId)
+                return true;
+
+            var mutualRows = await database.UserFollows
+                .AsNoTracking()
+                .CountAsync(f =>
+                    (f.FollowerId == viewer && f.FollowedId == userId) ||
+                    (f.FollowerId == userId && f.FollowedId == viewer),
+                    cancellationToken);
+
+            return mutualRows == 2;
+        }
+    }
+}