feat: metadata passthrough pada render

[ajianaz]

Sumber demand: Bannerbear metadata field — arbitrary string stored + returned. Use case pipeline: carry internal record ID / trace ID melalui render request.

Desain proposal:

• CLI/API request optional metadata: string (atau object kecil).
• API JSON response (response_format: json): echo balik metadata.
• CLI: tulis ke sidecar .json atau log terstruktur.

Acceptance:

• POST dengan metadata -> response JSON mengandung metadata persis sama.
• Tanpa metadata -> field absent (bukan null).

Effort: S. Prioritas #6.