sync: Missing origin-ownership check allows known peer to force-fetch into existing mirror #432

Summary

In crates/gitlawb-node/src/sync.rs:243-245 and crates/gitlawb-node/src/api/peers.rs:352-383, process_batch uses item.node_did solely to query resolve_origin_url. It does not verify that node_did matches the repository's recorded origin or owner DID.

Impact

As noted in PR #274's residuals, a well-formed slug matching an existing local mirror allows any known peer to enqueue a sync, repointing remote.origin and overwriting refs through the mirror refspec.

Remediation

1. Verify that item.node_did matches the repository's recorded origin DID before triggering fetch_repo.
2. Reject sync notifications where the notifying peer is not authorized for the repository slug.