feat: add asset variants table and URL selection

apps/web/src/__tests__/variant-url-404.test.ts

@@ -0,0 +1,9 @@
+import { test, expect } from "bun:test";
+
+test("/api/assets/:id/url returns 404 when requested variant missing", async () => {
+  const { pickVariantKey } = await import(
+    "../../app/api/assets/[id]/url/variant",
+  );
+  const key = pickVariantKey({ variants: [] }, { kind: "thumb", size: 256 });
+  expect(key).toBeNull();
+});