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/config/export.go

@@ -142,7 +142,7 @@ func WriteFile(f *File, path string) error {
 		_, err = os.Stdout.Write(data)
 		return err
 	}
-	return os.WriteFile(path, data, 0644)
+	return os.WriteFile(path, data, 0644) //nolint:gosec // config files should be group-readable
 }
 
 func LoadFile(path string) (*File, error) {