Build a hosted screenshot service for theme previews

[next-devin]

Follow-up to #34 and docs/production-capture-service.md.

Platform Engineering should build an authenticated Chromium service that accepts queued screenshot jobs and keeps tenant data separate.

The service should:

• only navigate to the store's network domain
• capture full-page PNGs at 1440px and 390px after fonts, images, and lazy media settle
• record theme and template revisions plus X-Theme-* headers
• store content-addressed artifacts per tenant behind expiring URLs
• enforce SSRF protection, rate limits, time and memory limits, retention, audit logs, and stable error codes

The design document describes the delivery slices. Until this service exists, local and CI capture remain the supported path.

[next-devin]

Closing after product review. A hosted capture service is more infrastructure than this workflow needs. Theme Kit will keep deterministic screenshots local and in CI.