atelier: map the platform, record what stays untouched, add sharp

no platform code references atelier and atelier references no platform code, so this extraction
has no reverse-dependency seam to design around. also records the database-separate decision
(collections and provider_settings both have rows on the live server) and adds sharp as a direct
dependency for the image-inspection code the sidecar needs — promoted from an existing transitive
resolution, no new package versions pulled in.

package.json

@@ -152,6 +152,7 @@
     "rehype-sanitize": "^6.0.0",
     "rehype-slug": "^6.0.0",
     "remark-gfm": "^4.0.1",
+    "sharp": "^0.33.5",
     "shiki": "^3.22.0",
     "sonner": "^2.0.7",
     "sounds": "workspace:*",