fix(tests): disable email in concurrency test to prevent CI timeouts

The TestNotificationManagerEmailConfigConcurrency test was causing CI
failures by triggering 1000+ email send attempts to a non-existent SMTP
server, each with retries and delays. This test verifies concurrent config
updates don't cause races, not actual email delivery. Disabling email
eliminates the network operations that were causing 60+ second test runs
and occasional CI failures.

internal/notifications/concurrency_test.go

@@ -21,8 +21,11 @@ func TestNotificationManagerEmailConfigConcurrency(t *testing.T) {
 	manager.SetGroupingWindow(0)
 	manager.SetCooldown(0)
 
+	// Disable email sending - this test verifies concurrent config updates
+	// don't cause races, not actual email delivery. Enabling email would
+	// trigger network operations with retries that slow down CI.
 	initialConfig := EmailConfig{
-		Enabled:  true,
+		Enabled:  false,
 		SMTPHost: "127.0.0.1",
 		SMTPPort: 2525,
 		From:     "initial@example.com",
@@ -39,7 +42,7 @@ func TestNotificationManagerEmailConfigConcurrency(t *testing.T) {
 		defer wg.Done()
 		for i := 0; i < iterations; i++ {
 			cfg := EmailConfig{
-				Enabled:  true,
+				Enabled:  false,
 				SMTPHost: "127.0.0.1",
 				SMTPPort: 2525,
 				From:     fmt.Sprintf("sender-%d@example.com", i),