fix(gateway): load the proxy routes and ticket settings in every environment

The YARP routes, the bucket path transform and the StreamTicket section only
existed in appsettings.Development.json, so a Production run had no routes at
all and every /media request fell through to a 404. Move them to
appsettings.json and leave only the environment-specific overrides (key path)
in the Development file; the container gets the filer address, the bucket and
the allowed CORS origins from environment variables.

backend/Musify.StreamingGateway/appsettings.json

@@ -6,5 +6,36 @@
       "Yarp": "Information"
     }
   },
-  "AllowedHosts": "*"
+  "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/" }
+        }
+      }
+    }
+  }
 }