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.Domain/Entities/Artist.cs

@@ -0,0 +1,32 @@
+using System.ComponentModel.DataAnnotations;
+using Musify.Domain.Abstractions;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    public class Artist : IAuditable
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        [MaxLength(200)]
+        public required string Name { get; set; }
+
+        [Required]
+        [MaxLength(200)]
+        public required string NormalizedName { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string ExternalId { get; set; }
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        public ICollection<TrackArtist> TrackArtists { get; set; } = new List<TrackArtist>();
+    }
+}