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/ValueObjects/AlbumPictures.cs

@@ -0,0 +1,13 @@
+namespace Musify.Domain.ValueObjects
+{
+    public class AlbumPictures
+    {
+        public string? OriginalName { get; set; }
+
+        public string? SmallName { get; set; }
+
+        public string? MediumName { get; set; }
+
+        public string? LargeName { get; set; }
+    }
+}

backend/Core/Musify.Domain/ValueObjects/LifeCycleStatus.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum LifeCycleStatus
+    {
+        Active = 0,
+        Removing = 1,
+        Failed = 2
+    }
+}

backend/Core/Musify.Domain/ValueObjects/MixItemSource.cs

@@ -0,0 +1,8 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum MixItemSource
+    {
+        Musify = 0,
+        YouTube = 1
+    }
+}

backend/Core/Musify.Domain/ValueObjects/PlayListPictures.cs

@@ -0,0 +1,19 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Domain.ValueObjects
+{
+    public class PlayListPictures
+    {
+        [Required]
+        public required string OriginalName { get; set; }
+
+        [Required]
+        public required string SmallName { get; set; }
+
+        [Required]
+        public required string MediumName { get; set; }
+
+        [Required]
+        public required string LargeName { get; set; }
+    }
+}

backend/Core/Musify.Domain/ValueObjects/ProcessingStatus.cs

@@ -0,0 +1,10 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum ProcessingStatus
+    {
+        Pending = 0,
+        Processing = 1,
+        Completed = 2,
+        Failed = 3
+    }
+}

backend/Core/Musify.Domain/ValueObjects/TrackAudio.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Musify.Domain.ValueObjects
+{
+#pragma warning disable CS8618
+    public class TrackAudio
+    {
+        public string? OriginalName { get; set; }
+
+        public string? FolderName { get; set; }
+
+        public ProcessingStatus TranscodeStatus { get; set; } = ProcessingStatus.Pending;
+
+        public bool DownloadRequested { get; set; }
+
+        public int RetryCount { get; set; }
+
+        public DateTime? LastRetryAt { get; set; }
+
+        [MemberNotNullWhen(true, nameof(FolderName))]
+        public bool IsProcessed => TranscodeStatus == ProcessingStatus.Completed;
+
+        public bool IsFailed => TranscodeStatus == ProcessingStatus.Failed;
+
+        public bool IsInProgress => TranscodeStatus is ProcessingStatus.Pending or ProcessingStatus.Processing;
+    }
+}

backend/Core/Musify.Domain/ValueObjects/TrackPictures.cs

@@ -0,0 +1,23 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Musify.Domain.ValueObjects
+{
+#pragma warning disable CS8618
+    public class TrackPictures
+    {
+        public string? OriginalName { get; set; }
+
+        public string? SmallName { get; set; }
+
+        public string? MediumName { get; set; }
+
+        public string? LargeName { get; set; }
+
+        public ProcessingStatus ProcessingStatus { get; set; } = ProcessingStatus.Pending;
+
+        [MemberNotNullWhen(true, nameof(SmallName))]
+        [MemberNotNullWhen(true, nameof(MediumName))]
+        [MemberNotNullWhen(true, nameof(LargeName))]
+        public bool IsProcessed => ProcessingStatus == ProcessingStatus.Completed;
+    }
+}

backend/Core/Musify.Domain/ValueObjects/TrackSource.cs

@@ -0,0 +1,8 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum TrackSource
+    {
+        Local = 0,
+        YouTube = 1
+    }
+}

backend/Core/Musify.Domain/ValueObjects/UploadIntentPurpose.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadIntentPurpose
+    {
+        PlayListPicture,
+        TrackPicture,
+        TrackAudio,
+    }
+}

backend/Core/Musify.Domain/ValueObjects/UploadIntentStatus.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadIntentStatus
+    {
+        Issued,
+        Consumed,
+        Expired,
+    }
+}

backend/Core/Musify.Domain/ValueObjects/UploadState.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadState
+    {
+        Pending,
+        Failed,
+        Successful,
+    }
+}