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.StreamingGateway/appsettings.json

@@ -0,0 +1,41 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning",
+      "Yarp": "Information"
+    }
+  },
+  "AllowedHosts": "*",
+
+  "StreamTicket": {
+    "PublicKeyPath": "",
+    "Audience": "media-gateway",
+    "Issuer": "musify-webapi",
+    "QueryParameterName": "t",
+    "MediaPathPrefix": "/media"
+  },
+
+  "Cors": {
+    "AllowedOrigins": []
+  },
+
+  "ReverseProxy": {
+    "Routes": {
+      "media": {
+        "ClusterId": "seaweed-filer",
+        "Match": { "Path": "/media/{**rest}" },
+        "Transforms": [
+          { "PathPattern": "/buckets/webapi-storage/{**rest}" }
+        ]
+      }
+    },
+    "Clusters": {
+      "seaweed-filer": {
+        "Destinations": {
+          "filer": { "Address": "http://localhost:8888/" }
+        }
+      }
+    }
+  }
+}