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/Tracks/CreateTrackRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Tracks;
+
+namespace Musify.Api.Validators.Tracks;
+
+public sealed class CreateTrackRequestValidator : AbstractValidator<CreateTrackRequest>
+{
+    public CreateTrackRequestValidator()
+    {
+        RuleFor(x => x.Title)
+            .NotEmpty()
+            .MaximumLength(200);
+
+        RuleFor(x => x.PictureIntentId)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioIntentId)
+            .NotEmpty();
+    }
+}

backend/Hosts/Musify.Api/Validators/Tracks/RequestTrackUploadUrlsRequestValidator.cs

@@ -0,0 +1,30 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Tracks;
+
+namespace Musify.Api.Validators.Tracks;
+
+public sealed class RequestTrackUploadUrlsRequestValidator : AbstractValidator<RequestTrackUploadUrlsRequest>
+{
+    public RequestTrackUploadUrlsRequestValidator()
+    {
+        RuleFor(x => x.PictureFileType)
+            .NotEmpty();
+
+        RuleFor(x => x.PictureContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioFileType)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.ExpectedPictureSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedPictureSizeBytes.HasValue);
+
+        RuleFor(x => x.ExpectedAudioSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedAudioSizeBytes.HasValue);
+    }
+}