build: update golangci-lint to v2.12.2 with canonical config

Replace the commit-pinned golangci-lint install refs in Dockerfile and
script/bootstrap with the v2.12.2 version pin, and replace .golangci.yml
with the canonical v2-schema config (lint settings now live under
linters.settings, so the lll/funlen/cyclop/dupl thresholds are actually
applied).

Fix all findings surfaced by the newly-applied config:

- goconst: introduce shared constants for repeated status, priority,
  and DNS fixture strings in watcher.go and the notify, state, and
  watcher tests
- dupl: consolidate duplicated ntfy/slack HTTP-error tests and
  SendNotification endpoint-error tests behind shared helpers
- lll: wrap long test table entries and comments; shorten one inline
  nolint justification

internal/watcher/watcher.go

@@ -26,6 +26,12 @@ const tlsPort = 443
 // hoursPerDay converts days to hours for duration calculations.
 const hoursPerDay = 24
 
+// Status values recorded for nameserver and certificate checks.
+const (
+	statusOK    = "ok"
+	statusError = "error"
+)
+
 // Params contains dependencies for Watcher.
 type Params struct {
 	fx.In
@@ -344,7 +350,7 @@ func buildHostnameState(
 	for ns, recs := range records {
 		hs.RecordsByNameserver[ns] = &state.NameserverRecordState{
 			Records:     recs,
-			Status:      "ok",
+			Status:      statusOK,
 			LastChecked: now,
 		}
 	}
@@ -402,7 +408,7 @@ func (w *Watcher) detectNSDisappearances(
 	current map[string]map[string][]string,
 ) {
 	for ns, prevNS := range prev.RecordsByNameserver {
-		if _, ok := current[ns]; ok || prevNS.Status != "ok" {
+		if _, ok := current[ns]; ok || prevNS.Status != statusOK {
 			continue
 		}
 
@@ -421,7 +427,7 @@ func (w *Watcher) detectNSDisappearances(
 
 	for ns := range current {
 		prevNS, ok := prev.RecordsByNameserver[ns]
-		if !ok || prevNS.Status != "error" {
+		if !ok || prevNS.Status != statusError {
 			continue
 		}
 
@@ -705,7 +711,7 @@ func (w *Watcher) handleTLSError(
 	now time.Time,
 	err error,
 ) {
-	if hasPrev && !w.firstRun && prev.Status == "ok" {
+	if hasPrev && !w.firstRun && prev.Status == statusOK {
 		msg := fmt.Sprintf(
 			"Host: %s\nIP: %s\nError: %s",
 			hostname, ip, err,
@@ -721,7 +727,7 @@ func (w *Watcher) handleTLSError(
 
 	w.state.SetCertificateState(
 		certKey, &state.CertificateState{
-			Status:      "error",
+			Status:      statusError,
 			Error:       err.Error(),
 			LastChecked: now,
 		},
@@ -748,7 +754,7 @@ func (w *Watcher) handleTLSSuccess(
 			Issuer:                  cert.Issuer,
 			NotAfter:                cert.NotAfter,
 			SubjectAlternativeNames: cert.SubjectAlternativeNames,
-			Status:                  "ok",
+			Status:                  statusOK,
 			LastChecked:             now,
 		},
 	)
@@ -760,7 +766,7 @@ 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,