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/metrics/prometheus.go

@@ -97,7 +97,7 @@ func Handler(eng *monitor.Engine) http.HandlerFunc {
 		}
 
 		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
-		w.Write([]byte(b.String()))
+		_, _ = w.Write([]byte(b.String())) //nolint:errcheck
 	}
 }