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/GetUserFollowers.cs

@@ -0,0 +1,46 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Shared;
+using Musify.Application.Users.Responses;
+using X.PagedList.EF;
+
+namespace Musify.Application.Users
+{
+    public record GetUserFollowersQuery(long? ViewerId, long UserId, int PageNumber, int PageSize)
+        : IQuery<ErrorOr<PaginatedResponse<UserSummaryResponse>>>;
+
+    public class GetUserFollowersQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserFollowersQuery, ErrorOr<PaginatedResponse<UserSummaryResponse>>>
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<UserSummaryResponse>>> Handle(GetUserFollowersQuery request, CancellationToken cancellationToken)
+        {
+            if (!await database.Users.AnyAsync(u => u.Id == request.UserId, cancellationToken))
+                return Error.NotFound();
+
+            if (!await FollowVisibility.CanViewFollowersAsync(database, request.ViewerId, request.UserId, cancellationToken))
+                return Error.Forbidden();
+
+            var followsQuery = database.UserFollows
+                .AsNoTracking()
+                .Where(f => f.FollowedId == request.UserId);
+
+            var totalCount = await followsQuery.CountAsync(cancellationToken);
+            var viewerId = request.ViewerId;
+
+            var page = await followsQuery
+                .OrderByDescending(f => f.CreatedAt)
+                .Select(f => new UserSummaryResponse(
+                    f.Follower.Id,
+                    f.Follower.Name,
+                    f.Follower.FirstName,
+                    f.Follower.SecondName,
+                    f.Follower.ProfilePictureUrl,
+                    viewerId != null && database.UserFollows.Any(v => v.FollowerId == viewerId && v.FollowedId == f.FollowerId)))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<UserSummaryResponse>.FromPagedList(page);
+        }
+    }
+}