Add newsletter image upload CLI/API, course banner resolution, and sponsor graphic auto-insert - #444

Summary

• Add POST /api/newsletter-issues/:id/images reusing savePublicImage (with full image decode and dimension validation from PR Validate sponsor graphic uploads, remove silent 300k slice, and alert on failures #440) to store durable public newsletter images in S3 (/api/public-images/:id).
• Add dataops newsletter image upload <issue-id> <path> CLI command (with --help and cli/skill/SKILL.md docs) that uploads an image and prints its durable public https:// URL.
• Update backend/src/newsletter/markdownPreview.ts to:
  • Render linked images ([![alt](src)](href)) as clickable <figure> elements.
  • Resolve known [Banner: Course name] markers (AI Dev Tools Zoomcamp, Machine Learning Zoomcamp) to their public course banners without inventing missing banners (Production AI Agents for Generative Media, event posters, recording thumbnails).
  • Auto-insert the linked sponsor doc's hosted graphicsUrl with its tracked ctaUrl into the SPONSORED block of the newsletter issue preview when no image is already present.
• Add cli/scripts/migrate-sponsor-doc-images.mjs to migrate data:image/...;base64,... sponsor graphicsUrl values to hosted storage (and support --replace <doc-id>=<path> for truncated legacy uploads).

Test Plan

• NODE_ENV=test FRONTEND_ROOT=../frontend SKIP_AUTH=true node --import tsx --test backend/tests/newsletter-markdown.test.ts backend/tests/public-images.test.ts (6/6 passing)
• npm --prefix backend run typecheck (passing)
• npm --prefix cli test (613/613 passing)