Add metrics for sync health monitoring

Add metrics to detect sync issues, fork divergence, and stuck states before they become critical.

Proposed Metrics

| Metric | Type | Description |
|------------------------------------------|-------|-----------------------------------------------------------------------------|
| zeam_forkchoice_nodes_total | Gauge | Number of blocks in forkchoice tree. Should grow over time. |
| zeam_cached_blocks_pending | Gauge | Blocks cached waiting for parents. Should stay low (<50). |
| zeam_sync_finalized_root_matches_peers | Gauge | 1 if our finalized root matches majority of peers, 0 otherwise. |
| zeam_sync_seconds_since_head_advance | Gauge | Seconds since head slot last increased. Alert if >60s when peers are ahead. |
| zeam_sync_seconds_since_finality_advance | Gauge | Seconds since finalized slot last increased. |
| zeam_sync_peer_finalized_slot_max | Gauge | Maximum finalized slot reported by any connected peer. |
| zeam_sync_slots_behind_peers | Gauge | max_peer_finalized_slot - our_finalized_slot. |