Add admin monitoring API endpoints for dashboard visibility
Four new authenticated endpoints expose existing internal state:
- getOverview: aggregated system status (network, replication, firehose, policy, verification)
- getDidStatus: per-DID detail (sync state, block/blob counts, peer endpoints, effective policy)
- getNetworkStatus: P2P connectivity (peerId, multiaddrs, connections)
- getPolicies: policy engine configuration and explicit DID lists