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/cluster/cluster.go

@@ -59,7 +59,7 @@ func runFollowerLoop(ctx context.Context, cfg Config, eng *monitor.Engine) {
 
 		if err == nil && resp.StatusCode == 200 {
 			isLeaderHealthy = true
-			resp.Body.Close()
+			_ = resp.Body.Close()
 		}
 
 		if isLeaderHealthy {