chore(build): bump Go toolchain to 1.27.0

Go 1.27.0 shipped on 2026-08-19. Raise the go directive and the builder
image so Docker and release builds pick it up; every CI job already reads
the version from go.mod, and golangci-lint v2.13.1 release binaries are
themselves built with go1.27.0, so the lint job needs no pin change.

internal/web/service/setting_mtls_test.go

@@ -187,7 +187,7 @@ func TestEnsureMasterClientCertConcurrentFirstUseMintsOneCredential(t *testing.T
 	}
 	close(start)
 	var first crypto.CertKeyPEM
-	for i := 0; i < callers; i++ {
+	for i := range callers {
 		credential := <-results
 		if err := <-errs; err != nil {
 			t.Fatalf("caller %d: %v", i, err)