feat: add asset variants table and URL selection

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

@@ -0,0 +1,9 @@
+export function pickVariantKey(
+  input: { variants: Array<{ kind: string; size: number; key: string }> },
+  req: { kind: string; size: number },
+) {
+  const v = input.variants.find(
+    (x) => x.kind === req.kind && x.size === req.size,
+  );
+  return v?.key ?? null;
+}