refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Application/Users/Handlers/SyncUserCommandHandler.cs

@@ -0,0 +1,50 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Users.Commands;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users.Handlers
+{
+    public class SyncUserCommandHandler(IDatabase database, ILogger<SyncUserCommandHandler> logger)
+        : ICommandHandler<SyncUserCommand, Result>
+    {
+        public async ValueTask<Result> Handle(SyncUserCommand request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users
+                .SingleOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
+
+            if (user is null)
+            {
+                await database.Users.AddAsync(new User
+                {
+                    Id = request.Id,
+                    Name = request.Name,
+                    NormalizedName = request.Name.ToUpperInvariant(),
+                    FirstName = request.FirstName,
+                    SecondName = request.SecondName,
+                    ProfilePictureUrl = request.ProfilePictureUrl
+                }, cancellationToken);
+
+                await database.SaveChangesAsync(cancellationToken);
+                logger.LogInformation("Provisioned user {UserId}", request.Id);
+
+                return Result.Success();
+            }
+
+            user.Name = request.Name;
+            user.NormalizedName = request.Name.ToUpperInvariant();
+            user.FirstName = request.FirstName;
+            user.SecondName = request.SecondName;
+            user.ProfilePictureUrl = request.ProfilePictureUrl;
+
+            var written = await database.SaveChangesAsync(cancellationToken);
+            if (written > 0)
+                logger.LogInformation("Synced profile for user {UserId}", request.Id);
+
+            return Result.Success();
+        }
+    }
+}