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/Mixes/GetMixById.cs

@@ -0,0 +1,30 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Mixes.Responses;
+
+namespace Musify.Application.Mixes
+{
+    public record GetMixByIdQuery(long UserId, Guid MixId)
+        : IQuery<ErrorOr<MixApplicationResponse>>;
+
+    public class GetMixByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetMixByIdQuery, ErrorOr<MixApplicationResponse>>
+    {
+        public async ValueTask<ErrorOr<MixApplicationResponse>> Handle(GetMixByIdQuery request, CancellationToken cancellationToken)
+        {
+            var mix = await database.Mixes
+                .AsNoTracking()
+                .Include(mix => mix.Items)
+                .SingleOrDefaultAsync(mix => mix.Id == request.MixId && mix.UserId == request.UserId, cancellationToken);
+
+            if (mix is null)
+                return Error.NotFound(description: $"Mix {request.MixId} was not found");
+
+            var items = mix.Items.OrderBy(item => item.Position).ToList();
+
+            return MixApplicationResponse.FromEntity(mix, items.Count, items);
+        }
+    }
+}