feat: add variation spawner and content repurposing

Variation spawner: select an image asset → spawn N variations that keep the
same emotional pattern/visual style but explore different hook angles. Runs
a focused ad-creative-designer agent with the original as a Gemini reference
image. New assets link back via parentAssetId.

Content repurposing: resize an image to all other platform dimensions using
Sharp (cover crop). Captions are re-toned for the target platform via Claude
CLI. No external APIs needed — fully local.

- Add parentAssetId self-relation to Asset model
- lib/repurpose.ts: Sharp resize, platform format mapping, caption re-toning
- lib/variations.ts: asset DNA extraction, variation prompt builder, mini-pipeline
- API routes: /api/assets/[id]/repurpose (GET formats, POST resize)
- API routes: /api/assets/[id]/variations (GET existing, POST spawn)
- Repurpose modal: checkbox list of target formats
- Variation modal: count picker, async launch
- Asset card: Repurpose + Variations buttons on image assets
- Asset lineage: "Derived from" shown on child assets

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/api/assets/[id]/variations/route.ts

@@ -0,0 +1,67 @@
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
+import { spawnVariations } from "@/lib/variations";
+import type { AppConfig } from "@/lib/claude";
+
+export async function GET(
+  _request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const session = await auth();
+  if (!session) return new Response("Unauthorized", { status: 401 });
+
+  const { id } = await params;
+
+  const variations = await prisma.asset.findMany({
+    where: { parentAssetId: id },
+    orderBy: { createdAt: "desc" },
+  });
+
+  return Response.json(variations);
+}
+
+export async function POST(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const session = await auth();
+  if (!session) return new Response("Unauthorized", { status: 401 });
+
+  const { id } = await params;
+  const body = await request.json();
+  const count = body.count || 5;
+
+  const asset = await prisma.asset.findUnique({
+    where: { id },
+    include: { campaign: { include: { app: true } } },
+  });
+
+  if (!asset) return Response.json({ error: "Not found" }, { status: 404 });
+  if (asset.type !== "image") {
+    return Response.json({ error: "Only image assets can spawn variations" }, { status: 400 });
+  }
+
+  // Build AppConfig if campaign has an app
+  let appConfig: AppConfig | undefined;
+  if (asset.campaign?.app) {
+    const app = asset.campaign.app;
+    appConfig = {
+      name: app.name,
+      slug: app.slug,
+      primaryColor: app.primaryColor,
+      accentColor: app.accentColor,
+      darkBg: app.darkBg,
+      assetsDir: `apps/${app.slug}`,
+      brandIdentity: app.brandIdentity,
+      productInfo: app.productInfo,
+      platformGuidelines: app.platformGuidelines,
+    };
+  }
+
+  // Launch async — don't block the request
+  spawnVariations({ assetId: id, count, appConfig }).catch((err) =>
+    console.error(`Variation spawning failed for asset ${id}:`, err)
+  );
+
+  return Response.json({ status: "spawning", count });
+}