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/Scalar/ScalarOptionsSetup.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Options;
+using Scalar.AspNetCore;
+using Musify.Api.Authentication;
+
+namespace Musify.Api.Scalar;
+
+public sealed class ScalarOptionsSetup(IOptions<AuthenticationConfiguration> options)
+    : IConfigureOptions<ScalarOptions>
+{
+    private const string SecuritySchemeId = "OAuth2";
+
+    private readonly AuthenticationConfiguration _configuration = options.Value;
+
+    public void Configure(ScalarOptions scalarOptions)
+    {
+        scalarOptions
+            .WithTitle("Musify API")
+            .AddPreferredSecuritySchemes(SecuritySchemeId)
+            .AddAuthorizationCodeFlow(SecuritySchemeId, flow =>
+            {
+                flow.ClientId = _configuration.ClientId;
+                flow.ClientSecret = _configuration.ClientSecret;
+                flow.AuthorizationUrl = _configuration.AuthorizationEndpoint;
+                flow.TokenUrl = _configuration.TokenEndpoint;
+                flow.Pkce = Pkce.Sha256;
+                flow.SelectedScopes = _configuration.Scopes;
+                flow.RedirectUri = _configuration.ScalarRedirectUri;
+                flow.RefreshUrl = _configuration.TokenEndpoint;
+            } )
+            .HideModels();
+    }
+}