fix: fallback to legacy keys for variant lookup

apps/web/app/api/assets/[id]/url/variant.ts

@@ -7,3 +7,25 @@ export function pickVariantKey(
   );
   return v?.key ?? null;
 }
+
+export function pickLegacyKeyForRequest(
+  input: {
+    asset: {
+      thumb_small_key: string | null;
+      thumb_med_key: string | null;
+      poster_key: string | null;
+    };
+  },
+  req: { kind: string; size: number },
+) {
+  if (req.kind === "thumb" && req.size === 256) {
+    return input.asset.thumb_small_key ?? null;
+  }
+  if (req.kind === "thumb" && req.size === 768) {
+    return input.asset.thumb_med_key ?? null;
+  }
+  if (req.kind === "poster" && req.size === 256) {
+    return input.asset.poster_key ?? null;
+  }
+  return null;
+}