Fix go vet IPv6 address format errors in email notifier and server

Replace fmt.Sprintf("%s:%d") with net.JoinHostPort() for IPv6 compatibility.
Bump setup-go action to v5 to resolve Node.js 20 deprecation warnings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Set up Go
-        uses: actions/setup-go@v4
+        uses: actions/setup-go@v5
         with:
           go-version: '1.22'
 

cmd/server/main.go

@@ -4,9 +4,11 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
+	"strconv"
 	"syscall"
 	"time"
 
@@ -241,7 +243,7 @@ func main() {
 	}
 
 	// Server configuration
-	addr := fmt.Sprintf("%s:%d", cfg.Server.Host, cfg.Server.Port)
+	addr := net.JoinHostPort(cfg.Server.Host, strconv.Itoa(cfg.Server.Port))
 	httpServer := &http.Server{
 		Addr:         addr,
 		Handler:      finalHandler,

internal/connector/notifier/email/email.go

@@ -8,6 +8,7 @@ import (
 	"log/slog"
 	"net"
 	"net/smtp"
+	"strconv"
 	"strings"
 	"time"
 
@@ -56,7 +57,7 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 		"smtp_port", cfg.SMTPPort)
 
 	// Test SMTP connectivity with timeout
-	addr := fmt.Sprintf("%s:%d", cfg.SMTPHost, cfg.SMTPPort)
+	addr := net.JoinHostPort(cfg.SMTPHost, strconv.Itoa(cfg.SMTPPort))
 	conn, err := net.DialTimeout("tcp", addr, 10*time.Second)
 	if err != nil {
 		return fmt.Errorf("failed to reach SMTP server %s: %w", addr, err)
@@ -123,7 +124,7 @@ func (c *Connector) SendEvent(ctx context.Context, event notifier.Event) error {
 // sendEmail sends an email message using the configured SMTP server.
 // It handles both TLS and plain authentication modes.
 func (c *Connector) sendEmail(ctx context.Context, to, subject, body string) error {
-	addr := fmt.Sprintf("%s:%d", c.config.SMTPHost, c.config.SMTPPort)
+	addr := net.JoinHostPort(c.config.SMTPHost, strconv.Itoa(c.config.SMTPPort))
 
 	// Connect to SMTP server
 	var auth smtp.Auth