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/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);
+}