chore: add golangci-lint config and fix all lint issues

Add .golangci.yml enabling errcheck, staticcheck, govet, gosec,
ineffassign, and unused linters. Fix 66 issues across 16 files:
- Check all unchecked errors (errcheck)
- Use HTTP status constants instead of numeric literals (staticcheck)
- Replace deprecated LineUp/LineDown with ScrollUp/ScrollDown (staticcheck)
- Convert sprintf+write patterns to fmt.Fprintf (staticcheck)
- Add ReadHeaderTimeout to http.Server (gosec)
- Remove unused types and functions (unused)
- Add nolint comments for intentional patterns (InsecureSkipVerify,
  math/rand for jitter, dialect-only SQL formatting)

internal/monitor/checker.go

@@ -131,7 +131,7 @@ func runPortCheck(site models.Site) CheckResult {
 	if err != nil {
 		return CheckResult{SiteID: site.ID, Status: "DOWN", LatencyNs: latency.Nanoseconds()}
 	}
-	conn.Close()
+	_ = conn.Close()
 	return CheckResult{SiteID: site.ID, Status: "UP", LatencyNs: latency.Nanoseconds()}
 }
 

internal/monitor/monitor.go

@@ -51,7 +51,7 @@ func NewEngine(s store.Store) *Engine {
 			Transport: &http.Transport{TLSClientConfig: &tls.Config{InsecureSkipVerify: false}},
 		},
 		insecureClient: &http.Client{
-			Transport: &http.Transport{TLSClientConfig: &tls.Config{InsecureSkipVerify: true}},
+			Transport: &http.Transport{TLSClientConfig: &tls.Config{InsecureSkipVerify: true}}, //nolint:gosec // intentional for IgnoreTLS sites
 		},
 	}
 }
@@ -279,7 +279,7 @@ func (e *Engine) ToggleSitePause(id int) bool {
 
 func (e *Engine) monitorRoutine(ctx context.Context, id int) {
 	// Stagger initial check to avoid thundering herd on startup
-	stagger := time.Duration(rand.IntN(3000)) * time.Millisecond
+	stagger := time.Duration(rand.IntN(3000)) * time.Millisecond //nolint:gosec // non-security jitter
 	select {
 	case <-time.After(stagger):
 	case <-ctx.Done():
@@ -323,7 +323,7 @@ func (e *Engine) monitorRoutine(ctx context.Context, id int) {
 		if interval < 5 {
 			interval = 5
 		}
-		jitter := time.Duration(rand.IntN(interval*100)) * time.Millisecond
+		jitter := time.Duration(rand.IntN(interval*100)) * time.Millisecond //nolint:gosec // non-security jitter
 		select {
 		case <-time.After(time.Duration(interval)*time.Second + jitter):
 		case <-ctx.Done():