Add ZMQ hashblock subscriber for sub-second chain refresh

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.

api/internal/state/aggregator.go

@@ -13,6 +13,7 @@ import (
 	"github.com/kamadopool/kamado-api/internal/bitcoind"
 	"github.com/kamadopool/kamado-api/internal/ckpool"
 	"github.com/kamadopool/kamado-api/internal/store"
+	"github.com/kamadopool/kamado-api/internal/zmqmon"
 )
 
 // Snapshot is the merged view served to the UI. All fields are safe to
@@ -81,8 +82,9 @@ func New(ck *ckpool.Client, rpc *bitcoind.RPC, interval time.Duration, log *slog
 // Run blocks until ctx is cancelled, refreshing the snapshot every Interval.
 // It runs one immediate refresh at startup so readers don't see an empty
 // snapshot after ctx launches the goroutine. Persisted block history is
-// loaded from the store before the first refresh.
-func (a *Aggregator) Run(ctx context.Context) {
+// loaded from the store before the first refresh. If tipEvents is non-nil,
+// each received tip triggers an immediate refresh outside the poll cadence.
+func (a *Aggregator) Run(ctx context.Context, tipEvents <-chan zmqmon.TipEvent) {
 	a.loadPersistedBlocks()
 	a.refresh(ctx)
 	t := time.NewTicker(a.Interval)
@@ -93,6 +95,13 @@ func (a *Aggregator) Run(ctx context.Context) {
 			return
 		case <-t.C:
 			a.refresh(ctx)
+		case ev, ok := <-tipEvents:
+			if !ok {
+				tipEvents = nil
+				continue
+			}
+			a.Log.Debug("zmq tip, refreshing", "hash", ev.Hash)
+			a.refresh(ctx)
 		}
 	}
 }