fix: clear golangci-lint backlog and make lint job blocking

Addresses errcheck, gosec, revive, staticcheck, and unused findings
across the codebase (unchecked error returns, unsafe file inclusion
warnings on operator/test-controlled paths, missing package comments,
unused parameters, deprecated API usage). Also fixes two suppression
comments that were silently no-ops due to wrong syntax (#nosec needs
a leading '#', nolint reasons need '//' not '--').

With the backlog clear, drop continue-on-error from the CI lint job
per the plan left in b4b4806.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/metrics/metrics.go

@@ -15,6 +15,7 @@ import (
 	"github.com/igodwin/notifier/internal/domain"
 	"github.com/igodwin/notifier/internal/logging"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -72,8 +73,8 @@ func NewCollector(service domain.NotificationService, queue domain.Queue, logger
 		c.queueDepth,
 		c.httpRequests,
 		c.httpDuration,
-		prometheus.NewGoCollector(),
-		prometheus.NewProcessCollector(prometheus.ProcessCollectorOpts{}),
+		collectors.NewGoCollector(),
+		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
 	)
 
 	return c