Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

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

@@ -0,0 +1,29 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Users.Responses;
+
+namespace Musify.Application.Users
+{
+    public record GetUserByIdQuery(long Id)
+        : IQuery<ErrorOr<UserApplicationResponse> >;
+
+    public class GetUserByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserByIdQuery, ErrorOr<UserApplicationResponse> >
+    {
+        public async ValueTask<ErrorOr<UserApplicationResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users
+                .AsNoTracking()
+                .Where(u => u.Id == request.Id)
+                .Select(u => UserApplicationResponse.FromEntity(u))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (user is null)
+                return Error.NotFound();
+
+            return user;
+        }
+    }
+}