Use fly.dev instead of .internal

`.internal` is not guaranteed to be routed to a live machine.
Theoretically, .flycast would be a better solution here, but sadly
updater backend has force_https set to true and flycast only works with
HTTP.

See: https://fly.io/docs/getting-started/app-services/

fly/Caddyfile

@@ -2,7 +2,7 @@
     root * /usr/share/caddy
     route {
         header /assets/* Cache-Control no-transform
-        reverse_proxy /api/* http://lineageos-updater-backend.internal:8080
+        reverse_proxy /api/* https://lineageos-updater-backend.fly.dev
         try_files {path} /index.html
         file_server
     }