feat(carousel): serve slide images from media, add picker to settings

[ssavutu]

Problem

The homepage carousel defaults pointed at /images/PodcastBanner.webp, /images/classifiedsBannerNew.webp, and /images/applyBannerNew.webp — files bundled in Scalene's public/images/, duplicated verbatim in Banner.astro's own fallback list.

• Editors couldn't change them. Swapping a banner meant a Scalene commit + redeploy, defeating the point of a CMS-editable carousel.
• Rename or drop a file on the Scalene side and the slide renders a broken <img>, with nothing in the CMS aware of it.
• The settings field was plain text with no preview, and a root-relative /images/... resolves against the CMS origin, which serves nothing there.

Changes

Images moved to the media filesystem. The three banners now live at /mnt/cephfs/media/wp-content/uploads/scalene/ and the Go defaults point at them via defaultCarouselImageBase. The prefix sits under wp-content/ because that is the only tree Nginx roots (deploy/nginx/triangle-cms.conf) — a top-level dir would fall through to the frontend proxy and 404 — and outside the YYYY/MM layout so the legacy WP corpus stays visibly separate.

Media picker in settings. The Image URL input becomes an Image field with a thumbnail, a Choose/Replace button opening the existing MediaPicker modal (search + upload, same as the article editor), and Clear. The raw URL input stays for external URLs and for the empty-image_url case that drives Scalene's one-hundred-slide text slide.

Notes

• No data migration needed. GET /v1/homepage returns a carousel byte-identical to the Go defaults, so no cms_settings row exists for homepage_carousel yet — the defaults are what the public homepage actually serves.
• Files are already live and independent of this merge: all three return 200 image/webp at the expected sizes.
• Don't overwrite these files in place. The /wp-content/ block stamps max-age=2592000, immutable, which assumes a filename pins its bytes — true for WP's size-suffixed derivatives, not for these. Replace under a new name and repoint the slide. Documented in a comment above the const.
• The three files aren't in the media library DB (copied directly, bypassing POST /v1/media), so they serve fine but won't appear in the MediaPicker grid until a "Reindex Media" run.
• Banner.astro's duplicated defaultSlides is intentionally untouched — it still falls back to the /images/ paths if the CMS returns an empty carousel.

Testing

go build, go vet, gofmt, go test -count=1 ./internal/..., tsc --noEmit, and npm run build all pass on this branch's base.

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