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()}
 }