build: update golangci-lint to v2.12.2 with commit-pinned installs

Pin golangci-lint to commit c0d3ddc9cf3faa61a4e378e879ece580256d76e5
(v2.12.2) in Dockerfile and script/bootstrap. Fix the goconst findings
the new version reports under the unchanged canonical .golangci.yml by
extracting shared test fixture constants and a statusError constant in
the watcher.

internal/watcher/watcher.go

@@ -26,6 +26,9 @@ const tlsPort = 443
 // hoursPerDay converts days to hours for duration calculations.
 const hoursPerDay = 24
 
+// statusError is the status value recorded for failed checks.
+const statusError = "error"
+
 // Params contains dependencies for Watcher.
 type Params struct {
 	fx.In
@@ -421,7 +424,7 @@ func (w *Watcher) detectNSDisappearances(
 
 	for ns := range current {
 		prevNS, ok := prev.RecordsByNameserver[ns]
-		if !ok || prevNS.Status != "error" {
+		if !ok || prevNS.Status != statusError {
 			continue
 		}
 
@@ -721,7 +724,7 @@ func (w *Watcher) handleTLSError(
 
 	w.state.SetCertificateState(
 		certKey, &state.CertificateState{
-			Status:      "error",
+			Status:      statusError,
 			Error:       err.Error(),
 			LastChecked: now,
 		},
@@ -760,7 +763,7 @@ func (w *Watcher) detectTLSChanges(
 	prev *state.CertificateState,
 	cert *tlscheck.CertificateInfo,
 ) {
-	if prev.Status == "error" {
+	if prev.Status == statusError {
 		msg := fmt.Sprintf(
 			"Host: %s\nIP: %s\nTLS recovered",
 			hostname, ip,