Add download, manifest, and url fields to module.json

- Added placeholders for url, manifest, and download fields
- These will be substituted by the release workflow with actual URLs
- Required for FoundryVTT to install the module from the package registry

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

module.json

@@ -26,5 +26,8 @@
       "path": "lang/en.json"
     }
   ],
+  "url": "${url}",
+  "manifest": "${manifest}",
+  "download": "${download}",
   "flags": {}
 }