Add thumbnail images to every remaining story card

[chellej]

Summary

• The insights grid already had real thumbnails; four other story-card groups still rendered with a blank colour-variant background and no image: the case-studies grid (and every "related cases" widget on the run-service pages, which reuses the same data), newsroom releases, partners, and the three sustainability pillars - 24 cards across 9 pages.
• Sourced a distinct, topically relevant free-license photo per card from Pexels (images.pexels.com CDN - no API key needed for direct image fetch), cropped server-side to 560x320 to match the existing insights-thumbs convention. Avoided images with visible third-party brand names/logos or identifiable close-up faces.
• Wired each set into its generator function the same way insights_main() was fixed previously: an image/image_alt field on the data list, rendered into an <img> inside the existing .story-card__media div. No CSS changes needed - object-fit: cover already handles populated vs. empty media.
• Case-study images are addressed by slug via a small case_thumb_path() helper, so the one asset set covers both the main case-studies.html grid and every service-detail page's related-cases section with no duplication.
• Removed the top-level assets/case-thumbs/ directory: it held 7 images under names that didn't match any of the 9 case-study slugs and was never referenced by the generator or the Makefile - dead weight from an earlier, incomplete pass at this same problem. The real case-study thumbnails now live under site/assets/case-thumbs/, matching where every other served asset directory already lives.

Test plan

• python3 -m py_compile scripts/generate-site.py
• python3 scripts/generate-site.py - clean run
• Ran the build a second time - zero additional diff (idempotent)
• CI link-checker's internal-link scan against the regenerated site/ - 0 missing links/images
• Visually confirmed all four affected pages (case studies, newsroom, partners, sustainability) via headless Chrome screenshots - every card shows a fitting, distinct thumbnail