Harden metadata sync placement and clock skew handling

[St4NNi]

Fix some minor sync placement and clock skew issues. It also prepares user scoped node types and excludes them from the default sync placement behavior

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.

[das-Abroxas]

Nice to have clock-skew protection to prevent (malicious) last-writer-wins metadata event conflicts. Also good, that User nodes are now exempt from sync peer selection as this could have lead to unpleasant experiences for a simple person participating in the network. lgtm.