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.Application/YouTube/GetYouTubeAlbum.cs

@@ -0,0 +1,39 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Services;
+
+namespace Musify.Application.YouTube
+{
+    public record GetYouTubeAlbumQuery(string AlbumId)
+        : IQuery<ErrorOr<YouTubeAlbumDetail>>;
+
+    public class GetYouTubeAlbumQueryHandler(
+        IYouTubeMusicService youTubeMusicService,
+        YouTubeTrackProvisioner provisioner,
+        ILogger<GetYouTubeAlbumQueryHandler> logger)
+        : IQueryHandler<GetYouTubeAlbumQuery, ErrorOr<YouTubeAlbumDetail>>
+    {
+        public async ValueTask<ErrorOr<YouTubeAlbumDetail>> Handle(GetYouTubeAlbumQuery request, CancellationToken cancellationToken)
+        {
+            if (string.IsNullOrEmpty(request.AlbumId))
+                return Error.Validation(description: "The album id must not be empty.");
+
+            var result = await youTubeMusicService.GetAlbumAsync(request.AlbumId, cancellationToken);
+            if (result.IsError)
+                return result;
+
+            try
+            {
+                await provisioner.MaterializeAlbumAsync(request.AlbumId, result.Value, cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogWarning(exception, "Failed to materialize YouTube album {AlbumId}", request.AlbumId);
+            }
+
+            return result;
+        }
+    }
+}

backend/Core/Musify.Application/YouTube/ResolveYouTubeTrackStream.cs

@@ -0,0 +1,76 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Services;
+using Musify.Application.YouTube.Responses;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.YouTube
+{
+    public record ResolveYouTubeTrackStreamCommand(string VideoId, long UserId)
+        : ICommand<ErrorOr<YouTubeStreamResponse>>;
+
+    public class ResolveYouTubeTrackStreamCommandHandler(
+        IDatabase database,
+        IYouTubeMusicService youTubeMusicService,
+        YouTubeTrackProvisioner provisioner,
+        TrackStreamIssuer streamIssuer,
+        ILogger<ResolveYouTubeTrackStreamCommandHandler> logger)
+        : ICommandHandler<ResolveYouTubeTrackStreamCommand, ErrorOr<YouTubeStreamResponse>>
+    {
+        public async ValueTask<ErrorOr<YouTubeStreamResponse>> Handle(ResolveYouTubeTrackStreamCommand request, CancellationToken cancellationToken)
+        {
+            var track = await provisioner.FindAsync(request.VideoId, cancellationToken);
+
+            if (track != null&& track.Audio.IsProcessed)
+            {
+                var serverStream = await streamIssuer.IssueAsync(track.Id, track.Audio.FolderName, request.UserId, cancellationToken);
+                return new YouTubeStreamResponse(
+                    track.Id,
+                    YouTubeStreamMode.Server,
+                    serverStream.ManifestUrl,
+                    serverStream.Ticket,
+                    serverStream.ExpiresInSeconds);
+            }
+
+            var streamInfo = await youTubeMusicService.GetAudioStreamAsync(request.VideoId, cancellationToken);
+            if (streamInfo.IsError)
+                return streamInfo.Errors;
+
+            if (track == null)
+            {
+                var provisionResult = await provisioner.GetOrCreateAsync(request.VideoId, cancellationToken);
+                if (!provisionResult.IsError)
+                    track = provisionResult.Value.Track;
+                else
+                    logger.LogWarning("Could not provision YouTube track {VideoId}: {Error}", request.VideoId, provisionResult.FirstError.Description);
+            }
+
+            if (track != null)
+            {
+                await database.ListeningHistories.AddAsync(new ListeningHistory
+                {
+                    UserId = request.UserId,
+                    TrackId = track.Id,
+                }, cancellationToken);
+
+                try
+                {
+                    await database.SaveChangesAsync(cancellationToken);
+                }
+                catch (DbUpdateException exception)
+                {
+                    logger.LogWarning(exception, "Failed to record listening history for YouTube video {VideoId}", request.VideoId);
+                }
+            }
+
+            return new YouTubeStreamResponse(
+                track?.Id,
+                YouTubeStreamMode.YouTube,
+                streamInfo.Value.Url, null,
+                streamInfo.Value.ExpiresInSeconds);
+        }
+    }
+}

backend/Core/Musify.Application/YouTube/Responses/YouTubeStreamResponse.cs

@@ -0,0 +1,15 @@
+namespace Musify.Application.YouTube.Responses
+{
+    public enum YouTubeStreamMode
+    {
+        Server = 0,
+        YouTube = 1
+    }
+
+    public record YouTubeStreamResponse(
+        Guid? TrackId,
+        YouTubeStreamMode Mode,
+        string StreamUrl,
+        string? Ticket,
+        int ExpiresInSeconds);
+}

backend/Core/Musify.Application/YouTube/SearchYouTubeAlbums.cs

@@ -0,0 +1,21 @@
+using ErrorOr;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.YouTube
+{
+    public record SearchYouTubeAlbumsQuery(string Query, string ContinuationToken)
+        : IQuery<ErrorOr<YouTubeAlbumSearchResult>>;
+
+    public class SearchYouTubeAlbumsQueryHandler(IYouTubeMusicService youTubeMusicService)
+        : IQueryHandler<SearchYouTubeAlbumsQuery, ErrorOr<YouTubeAlbumSearchResult>>
+    {
+        public async ValueTask<ErrorOr<YouTubeAlbumSearchResult>> Handle(SearchYouTubeAlbumsQuery request, CancellationToken cancellationToken)
+        {
+            if (string.IsNullOrEmpty(request.Query))
+                return Error.Validation(description: "The search query must not be empty.");
+
+            return await youTubeMusicService.SearchAlbumsAsync(request.Query, request.ContinuationToken, cancellationToken);
+        }
+    }
+}

backend/Core/Musify.Application/YouTube/SearchYouTubeTracks.cs

@@ -0,0 +1,21 @@
+using ErrorOr;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.YouTube
+{
+    public record SearchYouTubeTracksQuery(string Query, string ContinuationToken)
+        : IQuery<ErrorOr<YouTubeSearchResult>>;
+
+    public class SearchYouTubeTracksQueryHandler(IYouTubeMusicService youTubeMusicService)
+        : IQueryHandler<SearchYouTubeTracksQuery, ErrorOr<YouTubeSearchResult>>
+    {
+        public async ValueTask<ErrorOr<YouTubeSearchResult>> Handle(SearchYouTubeTracksQuery request, CancellationToken cancellationToken)
+        {
+            if (string.IsNullOrEmpty(request.Query))
+                return Error.Validation(description: "The search query must not be empty.");
+
+            return await youTubeMusicService.SearchSongsAsync(request.Query, request.ContinuationToken, cancellationToken);
+        }
+    }
+}