Kill ckpool on bitcoind failure so miners can failover

When bitcoind is unreachable for 3+ consecutive polls and no block
submission is pending, SIGTERM the ckpool process so miners disconnect
and failover to backup pools. Adds a red dashboard banner when
Bitcoin Core is down.

api/cmd/kamado-api/main.go

@@ -66,6 +66,7 @@ func main() {
 	agg.Store = blockStore
 	agg.MempoolBaseURL = cfg.MempoolBaseURL
 	agg.LogFilePath = cfg.CKPoolLogFile
+	agg.KillCKPool = killCKPool(log)
 
 	// Transaction accelerator (prioritisetransaction).
 	var accSvc *accelerator.Service
@@ -164,3 +165,38 @@ func main() {
 		os.Exit(1)
 	}
 }
+
+// killCKPool returns a function that finds the ckpool process by name
+// and sends it SIGTERM. Used by the aggregator to disconnect miners
+// when bitcoind is unreachable so they can failover to other pools.
+func killCKPool(log *slog.Logger) func() error {
+	return func() error {
+		entries, err := os.ReadDir("/proc")
+		if err != nil {
+			return fmt.Errorf("read /proc: %w", err)
+		}
+		for _, e := range entries {
+			if !e.IsDir() {
+				continue
+			}
+			pid, err := strconv.Atoi(e.Name())
+			if err != nil {
+				continue
+			}
+			cmdline, err := os.ReadFile(fmt.Sprintf("/proc/%d/cmdline", pid))
+			if err != nil {
+				continue
+			}
+			// ckpool's cmdline is NUL-separated; the first arg is the binary path.
+			if strings.Contains(string(cmdline), "ckpool") {
+				log.Info("sending SIGTERM to ckpool", "pid", pid)
+				proc, err := os.FindProcess(pid)
+				if err != nil {
+					return fmt.Errorf("find process %d: %w", pid, err)
+				}
+				return proc.Signal(syscall.SIGTERM)
+			}
+		}
+		return fmt.Errorf("ckpool process not found")
+	}
+}