Add cache_repodata toggle for github_rpm remotes

Cache generated repodata per replica and rebuild only when the remote's
rpm_metadata fingerprint (count, max id) changes.

docs/rpm/remote-github.md

@@ -53,6 +53,14 @@ derived metadata current off the client request path:
   ~once per `mutable_ttl` regardless of replica count, and the shared `etag`
   lets any replica issue the conditional request.
 
+### Repodata cache
+
+Repodata is regenerated from the metadata rows on every request by default. Set
+`cache_repodata: true` on the remote to keep the generated `repomd.xml` and its
+`primary`/`filelists`/`other` files in memory per replica. Each request checks a
+cheap row fingerprint (count + newest id) in the database and rebuilds only when
+it moved, so every replica picks up a scan's changes on its next request.
+
 ```hcl
 # Backend that serves the actual .rpm bytes from github.com.
 resource "artifactapi_remote_generic" "github" {