Make the Worker validate its service container and align Worker and Gateway hosts

The Worker now registers the Application layer, observability and stream tickets, validates the
container on build and ships a complete appsettings. The Gateway drops underscore fields and uses a
placeholder bucket in its default YARP transform.

backend/Hosts/Musify.StreamingGateway/appsettings.Development.json

@@ -1,5 +1,14 @@
 {
   "StreamTicket": {
     "PublicKeyPath": "../../../deploy/keys/stream_public.pem"
+  },
+  "ReverseProxy": {
+    "Routes": {
+      "media": {
+        "Transforms": [
+          { "PathPattern": "/buckets/webapi-storage/{**rest}" }
+        ]
+      }
+    }
   }
 }