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/Hosts/Musify.Api/Validators/Users/CreateUserRequestValidator.cs

@@ -0,0 +1,25 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Users;
+
+namespace Musify.Api.Validators.Users;
+
+public sealed class CreateUserRequestValidator : AbstractValidator<CreateUserRequest>
+{
+    public CreateUserRequestValidator()
+    {
+        RuleFor(x => x.Id)
+            .NotEmpty();
+
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .MaximumLength(48);
+
+        RuleFor(x => x.FirstName)
+            .MaximumLength(48)
+            .When(x => !string.IsNullOrWhiteSpace(x.FirstName));
+
+        RuleFor(x => x.SecondName)
+            .MaximumLength(48)
+            .When(x => !string.IsNullOrWhiteSpace(x.SecondName));
+    }
+}