Phase 2b: log tailer, block history, stdlib WebSocket push

Adds real-time block detection via ckpool log tailing and a push
channel for the upcoming Svelte UI, all stdlib-only:

- logmon.Tailer follows ckpool.log with rotation/truncation survival
  (inode + size tracking) and parses "Solved and confirmed block N"
  into BlockEvent values.
- state.Aggregator grows a 256-entry block ring, an OnRefresh hook,
  and IngestBlockEvents which best-effort enriches events with the
  block hash via bitcoind getblockhash.
- httpapi.Hub implements RFC 6455 from scratch (SHA1 handshake,
  unmasked text frames out, masked frames in, ping keepalive,
  per-client write mutex, slow-client drop) so we don't pull in a
  ws dependency before we can go mod tidy.
- New routes: GET /api/blocks and GET /api/ws. Snapshot pushes fire
  on every poll tick and immediately on block-solve.

ZMQ hashblock subscription and SQLite persistence are deferred to
Phase 2b.5 once the s9pk packaging repo exists and we have a real
build environment for adding Go deps.

api/cmd/kamado-api/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/kamadopool/kamado-api/internal/ckpool"
 	"github.com/kamadopool/kamado-api/internal/config"
 	"github.com/kamadopool/kamado-api/internal/httpapi"
+	"github.com/kamadopool/kamado-api/internal/logmon"
 	"github.com/kamadopool/kamado-api/internal/state"
 )
 
@@ -45,9 +46,19 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
+	api := httpapi.New(agg, log)
+
+	// Wire snapshot refreshes into the WebSocket hub so subscribers get
+	// real-time updates without polling.
+	agg.OnRefresh = api.Hub.Broadcast
+
 	go agg.Run(ctx)
 
-	api := httpapi.New(agg, log)
+	// Tail the ckpool log for block-solve events.
+	tailer := logmon.New(cfg.CKPoolLogFile, log)
+	go tailer.Run(ctx)
+	go agg.IngestBlockEvents(ctx, tailer.Events)
+
 	srv := &http.Server{
 		Addr:              cfg.ListenAddr,
 		Handler:           api.Handler(),