New internal/zmqmon package subscribes to bitcoind's hashblock
ZMQ topic and emits TipEvents. Uses github.com/go-zeromq/zmq4
(pure Go, builds with CGO_ENABLED=0). Exponential backoff on
connection failure so bitcoind restarts don't kill the
subscriber permanently; event channel drops rather than blocks
if the consumer is slow (signals are advisory, not logs).
Aggregator.Run now takes a <-chan TipEvent; when a tip arrives
it fires an immediate refresh() outside the normal ticker
cadence. With a 5s poll interval and 0.5-1s ZMQ latency from
bitcoind, dashboards now reflect new tips roughly 4x faster.
Endpoint comes from BITCOIN_ZMQ_BLOCK — empty disables ZMQ
entirely and the aggregator just runs on the ticker alone.
