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/DataTransferObjects/Albums/CreateAlbumRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.Albums;
+
+public record CreateAlbumRequest(
+    string Title,
+    string? Description,
+    int? ReleaseYear);

backend/Hosts/Musify.Api/DataTransferObjects/Albums/UpdateAlbumRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.Albums;
+
+public record UpdateAlbumRequest(
+    string NewTitle,
+    string? NewDescription,
+    int? NewReleaseYear);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/AddYouTubeTrackRequest.cs

@@ -0,0 +1,3 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record AddYouTubeTrackRequest(string VideoId);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/CreatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record CreatePlayListRequest(
+    string Name,
+    string? Description,
+    Guid? PictureIntentId);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/RequestPlayListPictureUploadRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record RequestPlayListPictureUploadRequest(
+    string FileType,
+    string ContentType,
+    long? ExpectedSizeBytes = null);

backend/Hosts/Musify.Api/DataTransferObjects/PlayLists/UpdatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record UpdatePlayListRequest(
+    string? NewName,
+    string? NewDescription,
+    Guid? NewPictureIntentId);

backend/Hosts/Musify.Api/DataTransferObjects/Tracks/CreateTrackRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.Tracks;
+
+public record CreateTrackRequest(
+    string Title,
+    Guid PictureIntentId,
+    Guid AudioIntentId);

backend/Hosts/Musify.Api/DataTransferObjects/Tracks/RequestTrackUploadUrlsRequest.cs

@@ -0,0 +1,9 @@
+namespace Musify.Api.DataTransferObjects.Tracks;
+
+public record RequestTrackUploadUrlsRequest(
+    string PictureFileType,
+    string PictureContentType,
+    string AudioFileType,
+    string AudioContentType,
+    long? ExpectedPictureSizeBytes = null,
+    long? ExpectedAudioSizeBytes = null);

backend/Hosts/Musify.Api/DataTransferObjects/Users/CreateUserRequest.cs

@@ -0,0 +1,7 @@
+namespace Musify.Api.DataTransferObjects.Users;
+
+public record CreateUserRequest(
+    long Id,
+    string Name,
+    string? FirstName,
+    string? SecondName);