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;
+        }
+    }
+}

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);
+        }
+    }
+}

backend/Core/Musify.Application/Users/GetUserFollowing.cs

@@ -0,0 +1,43 @@
+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 GetUserFollowingQuery(long? ViewerId, long UserId, int PageNumber, int PageSize)
+        : IQuery<ErrorOr<PaginatedResponse<UserSummaryResponse>>>;
+
+    public class GetUserFollowingQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserFollowingQuery, ErrorOr<PaginatedResponse<UserSummaryResponse>>>
+    {
+        public async ValueTask<ErrorOr<PaginatedResponse<UserSummaryResponse>>> Handle(GetUserFollowingQuery request, CancellationToken cancellationToken)
+        {
+            if (!await database.Users.AnyAsync(u => u.Id == request.UserId, cancellationToken))
+                return Error.NotFound();
+
+            var followsQuery = database.UserFollows
+                .AsNoTracking()
+                .Where(f => f.FollowerId == 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.Followed.Id,
+                    f.Followed.Name,
+                    f.Followed.FirstName,
+                    f.Followed.SecondName,
+                    f.Followed.ProfilePictureUrl,
+                    viewerId != null && database.UserFollows.Any(v => v.FollowerId == viewerId && v.FollowedId == f.FollowedId)))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return PaginatedResponse<UserSummaryResponse>.FromPagedList(page);
+        }
+    }
+}

backend/Core/Musify.Application/Users/GetUserProfile.cs

@@ -7,7 +7,7 @@ using Musify.Application.Serialization;
 
 namespace Musify.Application.Users
 {
-    public record GetUserProfileQuery(long UserId) : IQuery<ErrorOr<UserProfileResponse>>;
+    public record GetUserProfileQuery(long UserId, long? ViewerId = null) : IQuery<ErrorOr<UserProfileResponse>>;
 
     public record UserProfileResponse(
         [property: JsonConverter(typeof(LongAsStringConverter))] long Id,
@@ -17,7 +17,9 @@ namespace Musify.Application.Users
         string? ProfilePictureUrl,
         DateTime CreatedAt,
         int FollowersCount,
-        int FollowingCount);
+        int FollowingCount,
+        bool IsFollowing,
+        bool CanViewFollowers);
 
     public class GetUserProfileQueryHandler(IDatabase database)
         : IQueryHandler<GetUserProfileQuery, ErrorOr<UserProfileResponse>>
@@ -49,6 +51,14 @@ namespace Musify.Application.Users
                 .AsNoTracking()
                 .CountAsync(f => f.FollowerId == request.UserId, cancellationToken);
 
+            var isFollowing = request.ViewerId is { } viewerId && viewerId != request.UserId &&
+                await database.UserFollows
+                    .AsNoTracking()
+                    .AnyAsync(f => f.FollowerId == viewerId && f.FollowedId == request.UserId, cancellationToken);
+
+            var canViewFollowers = await FollowVisibility.CanViewFollowersAsync(
+                database, request.ViewerId, request.UserId, cancellationToken);
+
             return new UserProfileResponse(
                 user.Id,
                 user.Name,
@@ -57,7 +67,9 @@ namespace Musify.Application.Users
                 user.ProfilePictureUrl,
                 user.CreatedAt,
                 followersCount,
-                followingCount);
+                followingCount,
+                isFollowing,
+                canViewFollowers);
         }
     }
 }

backend/Core/Musify.Application/Users/GetUsers.cs

@@ -8,13 +8,13 @@ using Musify.Application.Contracts;
 
 namespace Musify.Application.Users
 {
-    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch)
-        : IQuery<ErrorOr<PaginatedResponse<UserApplicationResponse>>>;
+    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch, long? ViewerId = null)
+        : IQuery<ErrorOr<PaginatedResponse<UserSummaryResponse>>>;
 
     public class GetUsersQueryHandler(IDatabase database)
-        : IQueryHandler<GetUsersQuery, ErrorOr<PaginatedResponse<UserApplicationResponse>>>
+        : IQueryHandler<GetUsersQuery, ErrorOr<PaginatedResponse<UserSummaryResponse>>>
     {
-        public async ValueTask<ErrorOr<PaginatedResponse<UserApplicationResponse>>> Handle(GetUsersQuery request, CancellationToken cancellationToken)
+        public async ValueTask<ErrorOr<PaginatedResponse<UserSummaryResponse>>> Handle(GetUsersQuery request, CancellationToken cancellationToken)
         {
             var usersQuery = database.Users
                 .AsNoTracking()
@@ -27,11 +27,19 @@ namespace Musify.Application.Users
             }
 
             var totalCount = await usersQuery.CountAsync(cancellationToken);
+            var viewerId = request.ViewerId;
             var pagedUsers = await usersQuery
-                .Select(u => UserApplicationResponse.FromEntity(u))
+                .OrderBy(u => u.Name)
+                .Select(u => new UserSummaryResponse(
+                    u.Id,
+                    u.Name,
+                    u.FirstName,
+                    u.SecondName,
+                    u.ProfilePictureUrl,
+                    viewerId != null && database.UserFollows.Any(f => f.FollowerId == viewerId && f.FollowedId == u.Id)))
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
-            return PaginatedResponse<UserApplicationResponse>.FromPagedList(pagedUsers);
+            return PaginatedResponse<UserSummaryResponse>.FromPagedList(pagedUsers);
         }
     }
 }

backend/Core/Musify.Application/Users/Responses/UserSummaryResponse.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+using Musify.Application.Serialization;
+
+namespace Musify.Application.Users.Responses
+{
+    public record UserSummaryResponse(
+        [property: JsonConverter(typeof(LongAsStringConverter))] long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName,
+        string? ProfilePictureUrl,
+        bool IsFollowedByViewer);
+}