fix: route funkwhale cover images through internal media server

Cover proxy was fetching https://audio.giersig.eu/media/... but the
node jail has no outbound internet access. Adds mediaInternalUrl to
rewrite those fetches to http://10.100.0.10:8091/media/... instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

indiekit.config.mjs

@@ -376,6 +376,7 @@ export default {
     token: funkwhaleToken,
     cacheTtl: funkwhaleCacheTtl,
     syncInterval: funkwhaleSyncInterval,
+    mediaInternalUrl: "http://10.100.0.10:8091",
   },
   "@rmdes/indiekit-endpoint-lastfm": {
     mountPath: "/lastfmapi",