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/importer/kuma.go

@@ -177,7 +177,7 @@ func convertKumaMonitor(m KumaMonitor, alertMap map[int]int) models.Site {
 
 	for nidStr := range m.NotificationIDs {
 		var nid int
-		fmt.Sscanf(nidStr, "%d", &nid)
+		_, _ = fmt.Sscanf(nidStr, "%d", &nid) //nolint:errcheck
 		if upkeepID, ok := alertMap[nid]; ok {
 			site.AlertID = upkeepID
 			break