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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Hosts/Musify.Api/Validators/PlayLists/AddYouTubeTrackRequestValidator.cs

@@ -0,0 +1,14 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class AddYouTubeTrackRequestValidator : AbstractValidator<AddYouTubeTrackRequest>
+{
+    public AddYouTubeTrackRequestValidator()
+    {
+        RuleFor(x => x.VideoId)
+            .NotEmpty()
+            .Matches("^[A-Za-z0-9_-]{11}$");
+    }
+}

backend/Hosts/Musify.Api/Validators/PlayLists/CreatePlayListRequestValidator.cs

@@ -0,0 +1,17 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class CreatePlayListRequestValidator : AbstractValidator<CreatePlayListRequest>
+{
+    public CreatePlayListRequestValidator()
+    {
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .MaximumLength(50);
+
+        RuleFor(x => x.Description)
+            .MaximumLength(256);
+    }
+}

backend/Hosts/Musify.Api/Validators/PlayLists/RequestPlayListPictureUploadRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class RequestPlayListPictureUploadRequestValidator : AbstractValidator<RequestPlayListPictureUploadRequest>
+{
+    public RequestPlayListPictureUploadRequestValidator()
+    {
+        RuleFor(x => x.FileType)
+            .NotEmpty();
+
+        RuleFor(x => x.ContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.ExpectedSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedSizeBytes.HasValue);
+    }
+}

backend/Hosts/Musify.Api/Validators/PlayLists/UpdatePlayListRequestValidator.cs

@@ -0,0 +1,19 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class UpdatePlayListRequestValidator : AbstractValidator<UpdatePlayListRequest>
+{
+    public UpdatePlayListRequestValidator()
+    {
+        RuleFor(x => x.NewName)
+            .NotEmpty()
+            .MaximumLength(50)
+            .When(x => x.NewName is not null);
+
+        RuleFor(x => x.NewDescription)
+            .MaximumLength(256)
+            .When(x => x.NewDescription is not null);
+    }
+}