feat(metadata): allow admins to set a metadata source

The Ultimate edition ships preferences.json without gameMetadataSources, so
metadata sync was permanently unavailable there with no way to opt in.

Add an admin-only override (metadata.custom_source_enabled / _url) resolved
ahead of preferences, restricted to the two trusted FPFSS hosts and validated
on save. While it is off, the Metadata Update card explains that Ultimate does
not support metadata updates and that updating means a newer full package,
pointing advanced admins at the override. Enabling it requires confirming a
dialog that spells out what a sync rewrites.

docs/08-development/known-issues.md

@@ -32,6 +32,10 @@ on Infinity, because the files are already on disk.
 - With `ENABLE_LOCAL_DB_COPY=true`, the writes land on the container's copy and
   are wiped the next time the Launcher touches the source.
 
+**Status: implemented** (advanced override, edition-aware message, confirmation
+on enable). The remaining safety work — making the deletion pass opt-in and
+snapshotting `flashpoint.sqlite` before a sync — is still open.
+
 **Agreed approach:** the mixed library that results is acceptable, but the
 capability stays behind an explicit opt-in.