Harden metadata sync placement and clock skew handling

[St4NNi]

Summary

• Rebase selector-clock work onto current main without portal-serving changes.
• Make sync peer selection independent of the local node identity.
• Exclude user-device realm nodes from sync placement candidates and expose the node kind in info/admin config handling.
• Reject inbound metadata events stamped beyond MAX_CLOCK_SKEW_SECS to avoid permanent last-writer-wins conflicts.