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/Albums/AlbumReleaseYear.cs

@@ -0,0 +1,8 @@
+namespace Musify.Api.Validators.Albums;
+
+public static class AlbumReleaseYear
+{
+    public const int Earliest = 1877;
+
+    public static int Latest => DateTime.UtcNow.Year + 1;
+}

backend/Hosts/Musify.Api/Validators/Albums/CreateAlbumRequestValidator.cs

@@ -0,0 +1,21 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Albums;
+
+namespace Musify.Api.Validators.Albums;
+
+public sealed class CreateAlbumRequestValidator : AbstractValidator<CreateAlbumRequest>
+{
+    public CreateAlbumRequestValidator()
+    {
+        RuleFor(x => x.Title)
+            .NotEmpty()
+            .MaximumLength(200);
+
+        RuleFor(x => x.Description)
+            .MaximumLength(256);
+
+        RuleFor(x => x.ReleaseYear)
+            .InclusiveBetween(AlbumReleaseYear.Earliest, AlbumReleaseYear.Latest)
+            .When(x => x.ReleaseYear.HasValue);
+    }
+}

backend/Hosts/Musify.Api/Validators/Albums/UpdateAlbumRequestValidator.cs

@@ -0,0 +1,21 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Albums;
+
+namespace Musify.Api.Validators.Albums;
+
+public sealed class UpdateAlbumRequestValidator : AbstractValidator<UpdateAlbumRequest>
+{
+    public UpdateAlbumRequestValidator()
+    {
+        RuleFor(x => x.NewTitle)
+            .NotEmpty()
+            .MaximumLength(200);
+
+        RuleFor(x => x.NewDescription)
+            .MaximumLength(256);
+
+        RuleFor(x => x.NewReleaseYear)
+            .InclusiveBetween(AlbumReleaseYear.Earliest, AlbumReleaseYear.Latest)
+            .When(x => x.NewReleaseYear.HasValue);
+    }
+}