Add track coauthors with invitations, acceptance and coauthor-aware track responses

Owners invite existing users to coauthor a track; invitees accept or reject,
and either side can remove a coauthor. Only accepted coauthors appear in track
responses and in the coauthor's public track list. The owner also receives the
full invitation list with statuses on the user tracks response.

backend/Hosts/Musify.Api/Endpoints/UserEndpoints.cs

@@ -3,6 +3,8 @@ using Musify.Api.Authentication;
 using Musify.Api.Extensions;
 using Musify.Api.Filters;
 using Musify.Api.Models;
+using Musify.Application.Coauthors;
+using Musify.Application.Coauthors.Responses;
 using Musify.Application.Shared;
 using Musify.Application.Tracks;
 using Musify.Application.Tracks.Responses;
@@ -24,6 +26,14 @@ namespace Musify.Api.Endpoints
                 .WithSummary("Get paginated users")
                 .Produces<PaginatedResponse<UserSummaryResponse>>();
 
+            group.MapGet("/me/coauthor-invitations", GetCoauthorInvitations)
+                .AddEndpointFilter<ValidationFilter<PageQuery>>()
+                .WithName("GetCoauthorInvitations")
+                .WithSummary("Get your pending coauthor invitations with the track title and owner")
+                .RequireAuthorization()
+                .Produces<PaginatedResponse<CoauthorInvitationResponse>>()
+                .Produces(StatusCodes.Status401Unauthorized);
+
             group.MapGet("/{id:long}", GetUserById)
                 .WithName("GetUserById")
                 .WithSummary("Get a user by id")
@@ -108,6 +118,16 @@ namespace Musify.Api.Endpoints
             return Results.Ok(result);
         }
 
+        private static async Task<IResult> GetCoauthorInvitations(
+            IMediator mediator,
+            CurrentUser currentUser,
+            [AsParameters] PageQuery page,
+            CancellationToken cancellationToken)
+        {
+            var result = await mediator.Send(new GetCoauthorInvitationsQuery(currentUser.RequiredId, page.PageNumber, page.PageSize), cancellationToken);
+            return Results.Ok(result);
+        }
+
         private static async Task<IResult> GetUserById(
             IMediator mediator,
             long id,